diff --git a/src/cmd/go/internal/modfetch/proxy.go b/src/cmd/go/internal/modfetch/proxy.go
index 3971598..1c35d0b 100644
--- a/src/cmd/go/internal/modfetch/proxy.go
+++ b/src/cmd/go/internal/modfetch/proxy.go
@@ -171,6 +171,14 @@
 				fallBackOnError: fallBackOnError,
 			})
 		}
+
+		if len(proxyOnce.list) == 0 ||
+			len(proxyOnce.list) == 1 && proxyOnce.list[0].url == "noproxy" {
+			// There were no proxies, other than the implicit "noproxy" added when
+			// GONOPROXY is set. This can happen if GOPROXY is a non-empty string
+			// like "," or " ".
+			proxyOnce.err = fmt.Errorf("GOPROXY list is not the empty string, but contains no entries")
+		}
 	})
 
 	return proxyOnce.list, proxyOnce.err
@@ -191,7 +199,7 @@
 		return err
 	}
 	if len(proxies) == 0 {
-		return f("off")
+		panic("GOPROXY list is empty")
 	}
 
 	// We try to report the most helpful error to the user. "direct" and "noproxy"
diff --git a/src/cmd/go/testdata/script/mod_gonoproxy.txt b/src/cmd/go/testdata/script/mod_gonoproxy.txt
index d7848c7..a9e0ca4 100644
--- a/src/cmd/go/testdata/script/mod_gonoproxy.txt
+++ b/src/cmd/go/testdata/script/mod_gonoproxy.txt
@@ -18,6 +18,11 @@
 env GONOPROXY=none # that is, proxy all despite GOPRIVATE
 go get rsc.io/quote
 
+# When GOPROXY is not empty but contains no entries, an error should be reported.
+env GOPROXY=','
+! go get golang.org/x/text
+stderr '^go get golang.org/x/text: GOPROXY list is not the empty string, but contains no entries$'
+
 # When GOPROXY=off, fetching modules not matched by GONOPROXY fails.
 env GONOPROXY=*/fortune
 env GOPROXY=off