diff --git a/Modules/binascii.c b/Modules/binascii.c
index 95d7bb0..c74ed3c 100644
--- a/Modules/binascii.c
+++ b/Modules/binascii.c
@@ -351,7 +351,7 @@
 		if ( this_ch == BASE64_PAD )
 			npad++;
 		this_ch = table_a2b_base64[(*ascii_data) & 0x7f];
-		if ( this_ch == -1 ) continue;
+		if ( this_ch == (unsigned char) -1 ) continue;
 		/*
 		** Shift it in on the low end, and see if there's
 		** a byte ready for output.