diff --git a/src/zuc_modes.c b/src/zuc_modes.c
index ab386742..75f89b73 100644
--- a/src/zuc_modes.c
+++ b/src/zuc_modes.c
@@ -44,7 +44,7 @@ void zuc_eea_encrypt(const ZUC_UINT32 *in, ZUC_UINT32 *out, size_t nbits,
 	}
 
 	if (nbits % 32 != 0) {
-		out[nwords - 1] |= (0xffffffff << (32 - (nbits%32)));
+		out[nwords - 1] &= (0xffffffff << (32 - (nbits%32)));
 	}
 }