diff --git a/bin/expr/Makefile b/bin/expr/Makefile
index 23f768b28676..074980243b18 100644
--- a/bin/expr/Makefile
+++ b/bin/expr/Makefile
@@ -6,6 +6,9 @@ PROG=	expr
 SRCS=	expr.y
 YFLAGS=
 
+# expr relies on signed integer wrapping
+CFLAGS+= -fwrapv
+
 NO_WMISSING_VARIABLE_DECLARATIONS=
 
 .if ${MK_TESTS} != "no"