diff --git a/src/cmd/go/internal/modload/init.go b/src/cmd/go/internal/modload/init.go
index a9b77c82b3..1c31a5f90a 100644
--- a/src/cmd/go/internal/modload/init.go
+++ b/src/cmd/go/internal/modload/init.go
@@ -1018,7 +1018,7 @@ func keepSums(addDirect bool) map[module.Version]bool {
 			}
 		}
 		for _, pkg := range loaded.pkgs {
-			if pkg.testOf != nil || pkg.inStd {
+			if pkg.testOf != nil || pkg.inStd || module.CheckImportPath(pkg.path) != nil {
 				continue
 			}
 			for prefix := pkg.path; prefix != "."; prefix = path.Dir(prefix) {
diff --git a/src/cmd/go/testdata/script/mod_import_issue42891.txt b/src/cmd/go/testdata/script/mod_import_issue42891.txt
new file mode 100644
index 0000000000..a78cab29ba
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_import_issue42891.txt
@@ -0,0 +1,14 @@
+# If an import declaration is an absolute path, most commands should report
+# an error instead of going into an infinite loop.
+# Verifies golang.org/issue/42891.
+go list .
+stdout '^m$'
+
+-- go.mod --
+module m
+
+go 1.16
+-- m.go --
+package m
+
+import "/"