diff --git a/Changes b/Changes
index 0a56705..56eb739 100644
--- a/Changes
+++ b/Changes
@@ -5,6 +5,9 @@
       (diewald; AI-assisted Claude Opus 4.6)
     - [bugfix] Support regular expressions in attribute queries
       (fixes #80; diewald; AI-assisted Claude Opus 4.6)
+    - [bugfix] Fix StackOverflowError in ExpandedSpans by turning
+      a recursive call into a loop
+      (fixes #121; diewald; AI-assisted Claude Opus 4.6)
 
 0.64.6 2026-03-09
     - [performance] Add leaf cache. (diewald)
diff --git a/src/main/java/de/ids_mannheim/korap/query/spans/ExpandedSpans.java b/src/main/java/de/ids_mannheim/korap/query/spans/ExpandedSpans.java
index cb2bbd7..4349000 100644
--- a/src/main/java/de/ids_mannheim/korap/query/spans/ExpandedSpans.java
+++ b/src/main/java/de/ids_mannheim/korap/query/spans/ExpandedSpans.java
@@ -118,53 +118,63 @@
         CandidateSpan cs;
         int counter, start, end = 0;
 
+        boolean continueCandidateLoop = true;
+
         if (direction < 0) { // left
-            counter = max;
-            while (counter >= min) {
-                start = firstSpans.start() - counter;
-                if (start >= 0) {
-                    cs = new CandidateSpan(start, firstSpans.end(),
-                            firstSpans.doc(), firstSpans.cost(),
-                            createPayloads(start, firstSpans.start()));
+            while (continueCandidateLoop) {
+                continueCandidateLoop = false;
 
-                    candidateSpans.add(cs);
+                counter = max;
+                while (counter >= min) {
+                    start = firstSpans.start() - counter;
+                    if (start >= 0) {
+                        cs = new CandidateSpan(start, firstSpans.end(),
+                                firstSpans.doc(), firstSpans.cost(),
+                                createPayloads(start, firstSpans.start()));
+
+                        candidateSpans.add(cs);
+                    }
+                    counter--;
                 }
-                counter--;
-            }
 
-            int lastPosition = firstSpans.start();
-            if (hasMoreSpans && (hasMoreSpans = firstSpans.next())) {
-                start = Math.max(0, firstSpans.start() - max);
-                if (DEBUG) {
-                    log.debug("next candidate start: " + start + ", lastPosition "
-                              + lastPosition);
-                };
-                if (start <= lastPosition) {
-                    setCandidateList();
+                int lastPosition = firstSpans.start();
+                if (hasMoreSpans && (hasMoreSpans = firstSpans.next())) {
+                    start = Math.max(0, firstSpans.start() - max);
+                    if (DEBUG) {
+                        log.debug("next candidate start: " + start + ", lastPosition "
+                                  + lastPosition);
+                    };
+                    if (start <= lastPosition) {
+                        continueCandidateLoop = true;
+                    }
                 }
             }
         }
         else {
-            counter = min;
-            while (counter <= max) {
-                // TODO: How do I know if the end is already too far
-                // (over the end of the doc)?
-                end = firstSpans.end() + counter;
-                cs = new CandidateSpan(firstSpans.start(), end,
-                        firstSpans.doc(), firstSpans.cost(),
-                        createPayloads(firstSpans.end(), end));
-                candidateSpans.add(cs);
-                counter++;
-            }
+            while (continueCandidateLoop) {
+                continueCandidateLoop = false;
 
-            int lastPosition = end;
-            if (hasMoreSpans && (hasMoreSpans = firstSpans.next())) {
-                if (DEBUG) {
-                    log.debug("next candidate start: " + firstSpans.start()
-                              + ", lastPosition " + lastPosition);
-                };
-                if (firstSpans.start() <= lastPosition) {
-                    setCandidateList();
+                counter = min;
+                while (counter <= max) {
+                    // TODO: How do I know if the end is already too far
+                    // (over the end of the doc)?
+                    end = firstSpans.end() + counter;
+                    cs = new CandidateSpan(firstSpans.start(), end,
+                            firstSpans.doc(), firstSpans.cost(),
+                            createPayloads(firstSpans.end(), end));
+                    candidateSpans.add(cs);
+                    counter++;
+                }
+
+                int lastPosition = end;
+                if (hasMoreSpans && (hasMoreSpans = firstSpans.next())) {
+                    if (DEBUG) {
+                        log.debug("next candidate start: " + firstSpans.start()
+                                  + ", lastPosition " + lastPosition);
+                    };
+                    if (firstSpans.start() <= lastPosition) {
+                        continueCandidateLoop = true;
+                    }
                 }
             }
         }
diff --git a/src/test/java/de/ids_mannheim/korap/index/TestSpanExpansionIndex.java b/src/test/java/de/ids_mannheim/korap/index/TestSpanExpansionIndex.java
index 2f0fe80..dde8645 100644
--- a/src/test/java/de/ids_mannheim/korap/index/TestSpanExpansionIndex.java
+++ b/src/test/java/de/ids_mannheim/korap/index/TestSpanExpansionIndex.java
@@ -3,6 +3,8 @@
 import static de.ids_mannheim.korap.TestSimple.getJsonString;
 import static de.ids_mannheim.korap.TestSimple.simpleFieldDoc;
 import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertFalse;
+import static org.junit.Assert.assertTrue;
 
 import java.io.IOException;
 import java.util.Arrays;
@@ -731,7 +733,51 @@
         assertEquals(14, kr.getTotalResults());
 
     }
-    
+
+     // Issue #121:
+     // StackOverflowError in ExpandedSpans.setCandidateList()
+     // when searching [tt/p="NN"] []* [tt/p="NN"] on a large document.
+     @Test
+     public void testExpansionOverflowBug () throws IOException, QueryException {
+ 
+         FieldDocument fd = new FieldDocument();
+         fd.addString("ID", "doc-overflow");
+ 
+         int tokenCount = 5000;
+         StringBuilder surface = new StringBuilder();
+         StringBuilder annotation = new StringBuilder();
+ 
+         for (int i = 0; i < tokenCount; i++) {
+             String word = "tok" + i;
+             surface.append(word);
+             annotation.append("[(").append(i).append("-").append(i + 1).append(")");
+             annotation.append("s:").append(word);
+             annotation.append("|tt/p:NN");
+             if (i == 0) {
+                 annotation.append("|<>:base/s:t$<b>64<i>0<i>")
+                         .append(tokenCount).append("<i>")
+                         .append(tokenCount).append("<b>0");
+             }
+             annotation.append("|_").append(i).append("$<i>").append(i)
+                     .append("<i>").append(i + 1);
+             annotation.append("]");
+         }
+ 
+         fd.addTV("tokens", surface.toString(), annotation.toString());
+ 
+         KrillIndex ki = new KrillIndex();
+         ki.addDoc(fd);
+         ki.commit();
+ 
+         String json = getJsonString(getClass()
+                 .getResource("/queries/bugs/expansion_overflow.jsonld").getFile());
+         Krill krill = new Krill(json);
+ 
+         Result kr = krill.apply(ki);
+         assertFalse("Search should not produce errors", kr.hasErrors());
+         assertEquals("Should find matches", 499849, kr.getTotalResults());
+     }
+     
     
     private FieldDocument createFieldDoc6 () {
         FieldDocument fd = new FieldDocument();
diff --git a/src/test/java/de/ids_mannheim/korap/query/TestKrillQueryJSON.java b/src/test/java/de/ids_mannheim/korap/query/TestKrillQueryJSON.java
index 1d40690..a9758f0 100644
--- a/src/test/java/de/ids_mannheim/korap/query/TestKrillQueryJSON.java
+++ b/src/test/java/de/ids_mannheim/korap/query/TestKrillQueryJSON.java
@@ -856,4 +856,15 @@
             assertEquals("Query type is not supported", e.getMessage());
         };
     };
+
+    @Test
+    public void queryJSONexpansionOverflow () throws QueryException {
+        SpanQueryWrapper sqwi = getJsonQuery(
+                getClass().getResource("/queries/bugs/expansion_overflow.jsonld").getFile());
+
+        // [tt/p="NN"] []* [tt/p="NN"]
+        assertEquals(
+            "spanNext(SpanMultiTermQueryWrapper(tokens:/tt/p:NN/), spanExpansion(SpanMultiTermQueryWrapper(tokens:/tt/p:NN/), []{0, 100}, left))",
+            sqwi.toQuery().toString());
+    };
 };
diff --git a/src/test/resources/queries/bugs/expansion_overflow.jsonld b/src/test/resources/queries/bugs/expansion_overflow.jsonld
new file mode 100644
index 0000000..ec001cd
--- /dev/null
+++ b/src/test/resources/queries/bugs/expansion_overflow.jsonld
@@ -0,0 +1,44 @@
+{
+  "query": {
+    "operands": [
+      {
+        "@type": "koral:token",
+        "wrap": {
+          "foundry": "tt",
+          "@type": "koral:term",
+          "match": "match:eq",
+          "type": "type:regex",
+          "key": "NN",
+          "layer": "p"
+        }
+      },
+      {
+        "operands": [
+          {
+            "@type": "koral:token"
+          }
+        ],
+        "boundary": {
+          "min": 0,
+          "@type": "koral:boundary"
+        },
+        "@type": "koral:group",
+        "operation": "operation:repetition"
+      },
+      {
+        "@type": "koral:token",
+        "wrap": {
+          "foundry": "tt",
+          "@type": "koral:term",
+          "match": "match:eq",
+          "type": "type:regex",
+          "key": "NN",
+          "layer": "p"
+        }
+      }
+    ],
+    "@type": "koral:group",
+    "operation": "operation:sequence"
+  },
+  "@context": "http://korap.ids-mannheim.de/ns/koral/0.3/context.jsonld"
+}