diff --git a/bin/sh/arith_lex.l b/bin/sh/arith_lex.l
index 1113d2992970..b4a04d3fd3de 100644
--- a/bin/sh/arith_lex.l
+++ b/bin/sh/arith_lex.l
@@ -53,7 +53,15 @@ int yylex(void);
 
 #undef YY_INPUT
 #define YY_INPUT(buf,result,max) \
-	result = (*buf = *arith_buf++) ? 1 : YY_NULL;
+	do { \
+		result = strnlen(arith_buf, max); \
+		if (result == 0) \
+			result = YY_NULL; \
+		else { \
+			memcpy(buf, arith_buf, result); \
+			arith_buf += result; \
+		} \
+	} while (0);
 #define YY_NO_UNPUT
 #define YY_NO_INPUT
 %}