diff --git a/src/libImaging/ImPlatform.h b/src/libImaging/ImPlatform.h
index af9996ca9..f183c3aa4 100644
--- a/src/libImaging/ImPlatform.h
+++ b/src/libImaging/ImPlatform.h
@@ -61,7 +61,9 @@
 #if SIZEOF_LONG == 8
 #define INT64 long
 #elif SIZEOF_LONG_LONG == 8
-#define INT64 long
+#define INT64 long long
+#else
+#warning Cannot find required 64-bit integer type
 #endif
 
 #define INT8 signed char