diff --git a/src/cmd/go/internal/modload/init.go b/src/cmd/go/internal/modload/init.go
index af23647cd4..6f93b88eab 100644
--- a/src/cmd/go/internal/modload/init.go
+++ b/src/cmd/go/internal/modload/init.go
@@ -379,6 +379,10 @@ func InitMod(ctx context.Context) {
 		legacyModInit()
 	}
 
+	if err := checkModulePathLax(f.Module.Mod.Path); err != nil {
+		base.Fatalf("go: %v", err)
+	}
+
 	setDefaultBuildMod()
 	modFileToBuildList()
 	if cfg.BuildMod == "vendor" {
@@ -387,6 +391,49 @@ func InitMod(ctx context.Context) {
 	}
 }
 
+// checkModulePathLax checks that the path meets some minimum requirements
+// to avoid confusing users or the module cache. The requirements are weaker
+// than those of module.CheckPath to allow room for weakening module path
+// requirements in the future, but strong enough to help users avoid significant
+// problems.
+func checkModulePathLax(p string) error {
+	// TODO(matloob): Replace calls of this function in this CL with calls
+	// to module.CheckImportPath once it's been laxened, if it becomes laxened.
+	// See golang.org/issue/29101 for a discussion about whether to make CheckImportPath
+	// more lax or more strict.
+
+	errorf := func(format string, args ...interface{}) error {
+		return fmt.Errorf("invalid module path %q: %s", p, fmt.Sprintf(format, args...))
+	}
+
+	// Disallow shell characters " ' * < > ? ` | to avoid triggering bugs
+	// with file systems and subcommands. Disallow file path separators : and \
+	// because path separators other than / will confuse the module cache.
+	// See fileNameOK in golang.org/x/mod/module/module.go.
+	shellChars := "`" + `\"'*<>?|`
+	fsChars := `\:`
+	if i := strings.IndexAny(p, shellChars); i >= 0 {
+		return errorf("contains disallowed shell character %q", p[i])
+	}
+	if i := strings.IndexAny(p, fsChars); i >= 0 {
+		return errorf("contains disallowed path separator character %q", p[i])
+	}
+
+	// Ensure path.IsAbs and build.IsLocalImport are false, and that the path is
+	// invariant under path.Clean, also to avoid confusing the module cache.
+	if path.IsAbs(p) {
+		return errorf("is an absolute path")
+	}
+	if build.IsLocalImport(p) {
+		return errorf("is a local import path")
+	}
+	if path.Clean(p) != p {
+		return errorf("is not clean")
+	}
+
+	return nil
+}
+
 // fixVersion returns a modfile.VersionFixer implemented using the Query function.
 //
 // It resolves commit hashes and branch names to versions,
@@ -678,16 +725,35 @@ func findModulePath(dir string) (string, error) {
 	}
 
 	// Look for path in GOPATH.
+	var badPathErr error
 	for _, gpdir := range filepath.SplitList(cfg.BuildContext.GOPATH) {
 		if gpdir == "" {
 			continue
 		}
 		if rel := search.InDir(dir, filepath.Join(gpdir, "src")); rel != "" && rel != "." {
-			return filepath.ToSlash(rel), nil
+			path := filepath.ToSlash(rel)
+			// TODO(matloob): replace this with module.CheckImportPath
+			// once it's been laxened.
+			// Only checkModulePathLax here. There are some unpublishable
+			// module names that are compatible with checkModulePathLax
+			// but they already work in GOPATH so don't break users
+			// trying to do a build with modules. gorelease will alert users
+			// publishing their modules to fix their paths.
+			if err := checkModulePathLax(path); err != nil {
+				badPathErr = err
+				break
+			}
+			return path, nil
 		}
 	}
 
-	msg := `cannot determine module path for source directory %s (outside GOPATH, module path must be specified)
+	reason := "outside GOPATH, module path must be specified"
+	if badPathErr != nil {
+		// return a different error message if the module was in GOPATH, but
+		// the module path determined above would be an invalid path.
+		reason = fmt.Sprintf("bad module path inferred from directory in GOPATH: %v", badPathErr)
+	}
+	msg := `cannot determine module path for source directory %s (%s)
 
 Example usage:
 	'go mod init example.com/m' to initialize a v0 or v1 module
@@ -695,7 +761,7 @@ Example usage:
 
 Run 'go help mod init' for more information.
 `
-	return "", fmt.Errorf(msg, dir)
+	return "", fmt.Errorf(msg, dir, reason)
 }
 
 var (
diff --git a/src/cmd/go/testdata/script/mod_invalid_path.txt b/src/cmd/go/testdata/script/mod_invalid_path.txt
index 1ab418a075..05a5133571 100644
--- a/src/cmd/go/testdata/script/mod_invalid_path.txt
+++ b/src/cmd/go/testdata/script/mod_invalid_path.txt
@@ -1,12 +1,40 @@
-# Test that mod files with missing paths produce an error.
+# Test that mod files with invalid or missing paths produce an error.
 
 # Test that go list fails on a go.mod with no module declaration.
 cd $WORK/gopath/src/mod
 ! go list .
 stderr '^go: no module declaration in go.mod.\n\tRun ''go mod edit -module=example.com/mod'' to specify the module path.$'
 
+# Test that go mod init in GOPATH doesn't add a module declaration
+# with a path that can't possibly be a module path, because
+# it isn't even a valid import path.
+# The single quote and backtick are the only characters  we don't allow
+# in checkModulePathLax, but is allowed in a Windows file name.
+# TODO(matloob): choose a different character once
+# module.CheckImportPath is laxened and replaces
+# checkModulePathLax.
+cd $WORK/'gopath/src/m''d'
+! go mod init
+stderr 'cannot determine module path'
+
+# Test that a go.mod file is rejected when its module declaration has a path that can't
+# possibly be a module path, because it isn't even a valid import path
+cd $WORK/gopath/src/badname
+! go list .
+stderr 'invalid module path'
+
 -- mod/go.mod --
 
 -- mod/foo.go --
 package foo
 
+-- m'd/foo.go --
+package mad
+
+-- badname/go.mod --
+
+module .\.
+
+-- badname/foo.go --
+package badname
+