--- RunTestsPrebuilt.gmk
+++ RunTestsPrebuilt.gmk
@@ -214,15 +222,17 @@
 
 # Check number of cores
 ifeq ($(OPENJDK_TARGET_OS), linux)
-    NUM_CORES := $(shell $(CAT) /proc/cpuinfo  | $(GREP) -c processor)
+  NUM_CORES := $(shell $(CAT) /proc/cpuinfo  | $(GREP) -c processor)
 else ifeq ($(OPENJDK_TARGET_OS), macosx)
-    NUM_CORES := $(shell /usr/sbin/sysctl -n hw.ncpu)
+  NUM_CORES := $(shell /usr/sbin/sysctl -n hw.ncpu)
 else ifeq ($(OPENJDK_TARGET_OS), solaris)
-    NUM_CORES := $(shell LC_MESSAGES=C /usr/sbin/psrinfo -v | $(GREP) -c on-line)
+  NUM_CORES := $(shell LC_MESSAGES=C /usr/sbin/psrinfo -v | $(GREP) -c on-line)
 else ifeq ($(OPENJDK_TARGET_OS), windows)
-    NUM_CORES := $(NUMBER_OF_PROCESSORS)
-else
-    NUM_CORES := 1
+  NUM_CORES := $(NUMBER_OF_PROCESSORS)
+endif
+ifeq ($(NUM_CORES), )
+  $(warn Could not find number of CPUs, assuming 1)
+  NUM_CORES := 1
 endif
 
 ################################################################################