diff --git a/vm.c b/vm.c
index ec2f3176e6..2f6a244eee 100644
--- a/vm.c
+++ b/vm.c
@@ -2755,8 +2755,8 @@ check_machine_stack_size(size_t *sizep)
 #endif
 
 #ifdef PTHREAD_STACK_MIN
-    if (size < PTHREAD_STACK_MIN) {
-	*sizep = PTHREAD_STACK_MIN * 2;
+    if (size < (size_t)PTHREAD_STACK_MIN) {
+	*sizep = (size_t)PTHREAD_STACK_MIN * 2;
     }
 #endif
 }