diff --git a/src/net/http/request.go b/src/net/http/request.go
index 5251ebea665..aca55b1ca7b 100644
--- a/src/net/http/request.go
+++ b/src/net/http/request.go
@@ -779,7 +779,8 @@ func removeZone(host string) string {
 }
 
 // ParseHTTPVersion parses an HTTP version string.
-// "HTTP/1.0" returns (1, 0, true).
+// "HTTP/1.0" returns (1, 0, true). Note that strings without
+// a minor version, such as "HTTP/2", are not valid.
 func ParseHTTPVersion(vers string) (major, minor int, ok bool) {
 	const Big = 1000000 // arbitrary upper bound
 	switch vers {