diff --git a/lib/IDS/DeReKoVecs/derekovecs-server.c b/lib/IDS/DeReKoVecs/derekovecs-server.c
index 9d073a4..56e037c 100644
--- a/lib/IDS/DeReKoVecs/derekovecs-server.c
+++ b/lib/IDS/DeReKoVecs/derekovecs-server.c
@@ -526,8 +526,11 @@
     if (elem != NULL) {
       long j = (long)SvNV(*elem);
       AV *vector = newAV();
-      for (b = 0; b < size; b++) {
-        av_push(vector, newSVnv(M[b + j * size]));
+      /* ranks come from the request, reading outside the model would crash */
+      if (j >= 0 && j < words) {
+        for (b = 0; b < size; b++) {
+          av_push(vector, newSVnv(M[b + j * size]));
+        }
       }
       av_push(result, newRV_noinc((SV *)vector));
     }
@@ -535,6 +538,13 @@
   return newRV_noinc((SV *)result);
 }
 
+/* Word ids of the collocator db refer to the primary model, which occupies
+   [0, words - merged_end) of its own vocabulary. libcollocatordb crashes on
+   ids outside that range, and the ids come straight from the request. */
+int valid_cdb_node(long node) {
+  return cdb != NULL && node >= 0 && node < words - merged_end;
+}
+
 /* All functions handing a string back to perl return an SV*, because for a
    char* return value Inline::C only copies the string into the return SV and
    never frees the buffer we allocated here. */
@@ -544,7 +554,7 @@
   char pair_buffer[2048];
   buffer[0] = '[';
   buffer[1] = 0;
-  if (node >= sprofiles_qty) {
+  if (node < 0 || node >= sprofiles_qty) {
     printf("Not available in precomputed profile\n");
     return newSVpv("[{\"w\":\"not available\", \"v\":0}]\n", 0);
   }
@@ -566,15 +576,21 @@
 
 /* get_collocat*_as_json() hand out strdup()ed buffers that we own. */
 SV *getCollocationScores(long node, long collocate) {
-  char *json = (cdb ? (char *)get_collocation_scores_as_json(cdb, node, collocate) : NULL);
-  SV *res = newSVpv(json ? json : "[]", 0);
+  char *json = NULL;
+  SV *res;
+  if (valid_cdb_node(node) && valid_cdb_node(collocate))
+    json = (char *)get_collocation_scores_as_json(cdb, node, collocate);
+  res = newSVpv(json ? json : "{\"collocates\":[]}", 0);
   free(json);
   return res;
 }
 
 SV *getClassicCollocators(long node) {
-  char *json = (cdb ? (char *)get_collocators_as_json(cdb, node) : NULL);
-  SV *res = newSVpv(json ? json : "[]", 0);
+  char *json = NULL;
+  SV *res;
+  if (valid_cdb_node(node))
+    json = (char *)get_collocators_as_json(cdb, node);
+  res = newSVpv(json ? json : "{\"collocates\":[]}", 0);
   free(json);
   return res;
 }
@@ -1279,3 +1295,11 @@
 unsigned long getVocabSize() {
   return (unsigned long) words;
 }
+
+/* First rank of the primary model in the merged vocabulary, 0 if no second
+   model was merged in. mergeVectors() puts the merged in model at ranks
+   [0, merged_end) and the primary model - the one the collocator db belongs
+   to - at [merged_end, words). */
+long getMergedEnd() {
+  return (long) merged_end;
+}