diff --git a/rule_pyflakes.go b/rule_pyflakes.go
index 58ac4afcf..cfbc29a42 100644
--- a/rule_pyflakes.go
+++ b/rule_pyflakes.go
@@ -163,7 +163,7 @@ func (rule *RulePyflakes) parseNextError(stdout []byte, pos *Pos) ([]byte, error
 		msg = b[:idx]
 		b = b[idx+1:]
 	} else {
-		return nil, fmt.Errorf("error message from pyflakes does not end with \\n nor \\r\\n while checking script at %s. output: %q", pos, stdout)
+		return nil, fmt.Errorf(`error message from pyflakes does not end with \n nor \r\n while checking script at %s. output: %q`, pos, stdout)
 	}
 
 	// This method needs to be thread-safe since concurrentProcess.run calls its callback in a different goroutine.
diff --git a/rule_pyflakes_test.go b/rule_pyflakes_test.go
index 4021d49a7..e9ccc810b 100644
--- a/rule_pyflakes_test.go
+++ b/rule_pyflakes_test.go
@@ -1,6 +1,7 @@
 package actionlint
 
 import (
+	"strings"
 	"testing"
 )
 
@@ -82,3 +83,115 @@ func TestRulePyflakesDetectPythonShell(t *testing.T) {
 		})
 	}
 }
+
+func TestRulePyflakesParsePyflakesOutputOK(t *testing.T) {
+	tests := []struct {
+		what  string
+		input string
+		want  []string
+	}{
+		{
+			what:  "no error",
+			input: "",
+		},
+		{
+			what:  "ignore unrelated lines",
+			input: "this line\nshould be\nignored\n",
+		},
+		{
+			what:  "single error",
+			input: "<stdin>:1:7: undefined name 'foo'\n",
+			want: []string{
+				":1:2: pyflakes reported issue in this script: 1:7: undefined name 'foo' [pyflakes]",
+			},
+		},
+		{
+			what: "multiple errors",
+			input: "<stdin>:1:7: undefined name 'foo'\n" +
+				"<stdin>:1:7: undefined name 'foo'\n" +
+				"<stdin>:1:7: undefined name 'foo'\n",
+			want: []string{
+				":1:2: pyflakes reported issue in this script: 1:7: undefined name 'foo' [pyflakes]",
+				":1:2: pyflakes reported issue in this script: 1:7: undefined name 'foo' [pyflakes]",
+				":1:2: pyflakes reported issue in this script: 1:7: undefined name 'foo' [pyflakes]",
+			},
+		},
+		{
+			what: "syntax error",
+			input: "<stdin>:1:7: unexpected EOF while parsing\n" +
+				"print(\n" +
+				"      ^\n",
+			want: []string{
+				":1:2: pyflakes reported issue in this script: 1:7: unexpected EOF while parsing [pyflakes]",
+			},
+		},
+		{
+			what: "ignore unrelated lines between multiple errors",
+			input: "this line should be ignored\n" +
+				"this line should be ignored\n" +
+				"<stdin>:1:7: undefined name 'foo'\n" +
+				"this line should be ignored\n" +
+				"this line should be ignored\n" +
+				"<stdin>:1:7: undefined name 'foo'\n" +
+				"this line should be ignored\n" +
+				"this line should be ignored\n",
+			want: []string{
+				":1:2: pyflakes reported issue in this script: 1:7: undefined name 'foo' [pyflakes]",
+				":1:2: pyflakes reported issue in this script: 1:7: undefined name 'foo' [pyflakes]",
+			},
+		},
+		{
+			what: "CRLF",
+			input: "<stdin>:1:7: undefined name 'foo'\r\n" +
+				"<stdin>:1:7: undefined name 'foo'\r\n",
+			want: []string{
+				":1:2: pyflakes reported issue in this script: 1:7: undefined name 'foo' [pyflakes]",
+				":1:2: pyflakes reported issue in this script: 1:7: undefined name 'foo' [pyflakes]",
+			},
+		},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.what, func(t *testing.T) {
+			r := newRulePyflakes(&externalCommand{})
+			stdout := []byte(tc.input)
+			pos := &Pos{Line: 1, Col: 2}
+			for len(stdout) > 0 {
+				o, err := r.parseNextError(stdout, pos)
+				if err != nil {
+					t.Fatalf("Parse error %q while reading input %q", err, stdout)
+				}
+				stdout = o
+			}
+			have := r.Errs()
+			if len(have) != len(tc.want) {
+				msgs := []string{}
+				for _, e := range have {
+					msgs = append(msgs, e.Error())
+				}
+				t.Fatalf("%d errors were expected but got %d errors. got errors are:\n%#v", len(tc.want), len(have), msgs)
+			}
+
+			for i, want := range tc.want {
+				have := have[i]
+				msg := have.Error()
+				if !strings.Contains(msg, want) {
+					t.Errorf("Error %q does not contain expected message %q", msg, want)
+				}
+			}
+		})
+	}
+}
+
+func TestRulePyflakesParsePyflakesOutputError(t *testing.T) {
+	r := newRulePyflakes(&externalCommand{})
+	_, err := r.parseNextError([]byte("<stdin>:1:7: undefined name 'foo'"), &Pos{})
+	if err == nil {
+		t.Fatal("Error did not happen")
+	}
+	have := err.Error()
+	want := `error message from pyflakes does not end with \n nor \r\n`
+	if !strings.Contains(have, want) {
+		t.Fatalf("Error %q does not contain expected message %q", have, want)
+	}
+}