diff --git a/vm_insnhelper.c b/vm_insnhelper.c
index 4a6d7dc9d6..cb3236b1bc 100644
--- a/vm_insnhelper.c
+++ b/vm_insnhelper.c
@@ -1406,7 +1406,7 @@ FIXNUM_2_P(VALUE a, VALUE b)
 static bool
 FLONUM_2_P(VALUE a, VALUE b)
 {
-#ifdef USE_FLONUM
+#if USE_FLONUM
     /* FLONUM_P(a) && FLONUM_P(b)
      * == ((a & 3) == 2) && ((b & 3) == 2)
      * == ! ((a ^ 2) | (b ^ 2) & 3)