diff --git a/src/fmt/fmt_test.go b/src/fmt/fmt_test.go
index c06f9a1fcf..5d3357004f 100644
--- a/src/fmt/fmt_test.go
+++ b/src/fmt/fmt_test.go
@@ -537,6 +537,7 @@ var fmtTests = []struct {
 	{"%s", nil, "%!s(<nil>)"},
 	{"%T", nil, "<nil>"},
 	{"%-1", 100, "%!(NOVERB)%!(EXTRA int=100)"},
+	{"%017091901790959340919092959340919017929593813360", 0, "%!(NOVERB)%!(EXTRA int=0)"},
 
 	// The "<nil>" show up because maps are printed by
 	// first obtaining a list of keys and then looking up
diff --git a/src/fmt/print.go b/src/fmt/print.go
index 8e35a890ec..9c373145dd 100644
--- a/src/fmt/print.go
+++ b/src/fmt/print.go
@@ -292,6 +292,9 @@ func parsenum(s string, start, end int) (num int, isnum bool, newi int) {
 	}
 	for newi = start; newi < end && '0' <= s[newi] && s[newi] <= '9'; newi++ {
 		num = num*10 + int(s[newi]-'0')
+		if num < 0 {
+			return 0, false, end // Overflow; crazy long number most likely.
+		}
 		isnum = true
 	}
 	return