diff --git a/Makefile b/Makefile
index 8f6ea60..b48b0f7 100644
--- a/Makefile
+++ b/Makefile
@@ -7,7 +7,8 @@
 MAX_THREADS ?= $(shell nproc)
 YY ?= 18
 MAKE ?= make -j $(shell nproc)
-KORAPXML2CONLLU ?= java -Xmx$(shell $$((1625 * $$(nproc))))m -jar lib/korapxml2conllu.jar
+KORAPXML2CONLLU_HEAP ?= $(shell echo "$$(($$(nproc) * 1625))")
+KORAPXML2CONLLU ?= java -Xmx$(KORAPXML2CONLLU_HEAP)m -jar lib/korapxml2conllu.jar
 SAXON ?= java -cp lib/saxon9ee.jar:lib/xml-resolver-1.2.jar net.sf.saxon.Transform -expand:off -catalog:"lib/dtds/xhtml11/xhtmlcatalog.xml;lib/dtds/xhtml/dtd/xhtmlcatalog.xml"
 
 .PHONY: all clean test i5 i5valid krill index deploy server-log server-status