diff --git a/xpra/codecs/webp/encoder.pyx b/xpra/codecs/webp/encoder.pyx
index 8768b05c94..6486b6f126 100644
--- a/xpra/codecs/webp/encoder.pyx
+++ b/xpra/codecs/webp/encoder.pyx
@@ -398,7 +398,10 @@ def encode(image, int quality=50, int speed=50, supports_alpha=False, content_ty
     pixels = image.get_pixels()
     cdef int threshold_delta = -int(content_type=="text")*20
     cdef int alpha_int = int(supports_alpha and pixel_format.find("A")>=0)
-    cdef int use_argb = int(quality>=(SUBSAMPLING_THRESHOLD+threshold_delta))
+    #we can only use YUV420P subsampling if we are 100% certain
+    #that the RGB pixel data contains a valid alpha component
+    #because WebPPictureARGBToYUVA will set the alpha channel if it finds ANY value
+    cdef int use_argb = int(quality>=(SUBSAMPLING_THRESHOLD+threshold_delta)) or pixel_format.find("A")<0
 
     cdef WebPConfig config
     cdef WebPPreset preset = DEFAULT_PRESET