diff --git a/simpc-mode.el b/simpc-mode.el
index fc0e5f2..928bb93 100644
--- a/simpc-mode.el
+++ b/simpc-mode.el
@@ -24,7 +24,7 @@
     "char16_t" "char32_t" "char8_t"
     "int8_t" "uint8_t" "int16_t" "uint16_t" "int32_t" "uint32_t" "int64_t" "uint64_t"
     "uintptr_t"
-    "size_t"
+    "size_t" "ptrdiff_t"
     "va_list"))
 
 (defun simpc-keywords ()