diff --git a/src/cmd/go/internal/modload/import.go b/src/cmd/go/internal/modload/import.go
index 5906d64..d7fca8f 100644
--- a/src/cmd/go/internal/modload/import.go
+++ b/src/cmd/go/internal/modload/import.go
@@ -184,8 +184,9 @@
 		if !pathIsStd {
 			if cfg.BuildModReason == "" {
 				queryErr = fmt.Errorf("import lookup disabled by -mod=%s", cfg.BuildMod)
+			} else {
+				queryErr = fmt.Errorf("import lookup disabled by -mod=%s\n\t(%s)", cfg.BuildMod, cfg.BuildModReason)
 			}
-			queryErr = fmt.Errorf("import lookup disabled by -mod=%s\n\t(%s)", cfg.BuildMod, cfg.BuildModReason)
 		}
 		return module.Version{}, "", &ImportMissingError{Path: path, QueryErr: queryErr}
 	}
diff --git a/src/cmd/go/testdata/script/mod_readonly.txt b/src/cmd/go/testdata/script/mod_readonly.txt
index 77fc735..751f6e6 100644
--- a/src/cmd/go/testdata/script/mod_readonly.txt
+++ b/src/cmd/go/testdata/script/mod_readonly.txt
@@ -7,6 +7,7 @@
 cp go.mod go.mod.empty
 ! go list all
 stderr '^can''t load package: x.go:2:8: cannot find module providing package rsc\.io/quote: import lookup disabled by -mod=readonly'
+! stderr '\(\)' # If we don't have a reason for -mod=readonly, don't log an empty one.
 cmp go.mod go.mod.empty
 
 # -mod=readonly should be set implicitly if the go.mod file is read-only