diff --git a/src/libImaging/Convert.c b/src/libImaging/Convert.c
index 54ac02b6c..5a8e1ac73 100644
--- a/src/libImaging/Convert.c
+++ b/src/libImaging/Convert.c
@@ -1706,7 +1706,7 @@ ImagingConvertTransparent(Imaging imIn, const char *mode, int r, int g, int b) {
         return (Imaging)ImagingError_ModeError();
     }
 
-    int rgbIn = strcmp(imIn->mode, "rgb") == 0;
+    int rgbIn = strcmp(imIn->mode, "RGB") == 0;
     int bitIn = strcmp(imIn->mode, "1") == 0;
     int iIn = strcmp(imIn->mode, "I") == 0;
     int lIn = strcmp(imIn->mode, "L") == 0;