diff --git a/dll/win32/winmm/mmio.c b/dll/win32/winmm/mmio.c
index a0043d6627e..5e86722e747 100644
--- a/dll/win32/winmm/mmio.c
+++ b/dll/win32/winmm/mmio.c
@@ -366,8 +366,8 @@ static FOURCC MMIO_ParseExtA(LPCSTR szFileName)
     FOURCC ret = 0;
 
     /* Note that ext{Start,End} point to the . and + respectively */
-    LPSTR extEnd;
-    LPSTR extStart;
+    LPCSTR extEnd;
+    LPCSTR extStart;
 
     CHAR ext[5];