diff --git a/Modules/binascii.c b/Modules/binascii.c
index e5418a4..648088b 100644
--- a/Modules/binascii.c
+++ b/Modules/binascii.c
@@ -653,7 +653,7 @@
                 /* More than 3 in a row. Output RLE. */
                 *out_data++ = ch;
                 *out_data++ = RUNCHAR;
-                *out_data++ = inend-in;
+                *out_data++ = (unsigned char) inend-in;
                 in = inend-1;
             } else {
                 /* Less than 3. Output the byte itself */