diff --git a/signature/internal/rekor_set.go b/signature/internal/rekor_set.go
index e79c91cf99..e065b405c0 100644
--- a/signature/internal/rekor_set.go
+++ b/signature/internal/rekor_set.go
@@ -40,17 +40,20 @@ type UntrustedRekorPayload struct {
 // A compile-time check that UntrustedRekorSET implements json.Unmarshaler
 var _ json.Unmarshaler = (*UntrustedRekorSET)(nil)
 
-// UnmarshalJSON implements the json.Unmarshaler interface
-func (s *UntrustedRekorSET) UnmarshalJSON(data []byte) error {
-	err := s.strictUnmarshalJSON(data)
-	if err != nil {
-		if formatErr, ok := err.(JSONFormatError); ok {
-			err = NewInvalidSignatureError(formatErr.Error())
-		}
+// JSONFormatToInvalidSignatureError converts JSONFormatError to InvalidSignatureError.
+// All other errors are returned as is.
+func JSONFormatToInvalidSignatureError(err error) error {
+	if formatErr, ok := err.(JSONFormatError); ok {
+		err = NewInvalidSignatureError(formatErr.Error())
 	}
 	return err
 }
 
+// UnmarshalJSON implements the json.Unmarshaler interface
+func (s *UntrustedRekorSET) UnmarshalJSON(data []byte) error {
+	return JSONFormatToInvalidSignatureError(s.strictUnmarshalJSON(data))
+}
+
 // strictUnmarshalJSON is UnmarshalJSON, except that it may return the internal JSONFormatError error type.
 // Splitting it into a separate function allows us to do the JSONFormatError → InvalidSignatureError in a single place, the caller.
 func (s *UntrustedRekorSET) strictUnmarshalJSON(data []byte) error {
@@ -77,13 +80,7 @@ var _ json.Unmarshaler = (*UntrustedRekorPayload)(nil)
 
 // UnmarshalJSON implements the json.Unmarshaler interface
 func (p *UntrustedRekorPayload) UnmarshalJSON(data []byte) error {
-	err := p.strictUnmarshalJSON(data)
-	if err != nil {
-		if formatErr, ok := err.(JSONFormatError); ok {
-			err = NewInvalidSignatureError(formatErr.Error())
-		}
-	}
-	return err
+	return JSONFormatToInvalidSignatureError(p.strictUnmarshalJSON(data))
 }
 
 // strictUnmarshalJSON is UnmarshalJSON, except that it may return the internal JSONFormatError error type.
diff --git a/signature/internal/sigstore_payload.go b/signature/internal/sigstore_payload.go
index a2609c954b..5ee1156793 100644
--- a/signature/internal/sigstore_payload.go
+++ b/signature/internal/sigstore_payload.go
@@ -79,13 +79,7 @@ var _ json.Unmarshaler = (*UntrustedSigstorePayload)(nil)
 
 // UnmarshalJSON implements the json.Unmarshaler interface
 func (s *UntrustedSigstorePayload) UnmarshalJSON(data []byte) error {
-	err := s.strictUnmarshalJSON(data)
-	if err != nil {
-		if formatErr, ok := err.(JSONFormatError); ok {
-			err = NewInvalidSignatureError(formatErr.Error())
-		}
-	}
-	return err
+	return JSONFormatToInvalidSignatureError(s.strictUnmarshalJSON(data))
 }
 
 // strictUnmarshalJSON is UnmarshalJSON, except that it may return the internal JSONFormatError error type.
@@ -126,7 +120,7 @@ func (s *UntrustedSigstorePayload) strictUnmarshalJSON(data []byte) error {
 	if gotTimestamp {
 		intTimestamp := int64(timestamp)
 		if float64(intTimestamp) != timestamp {
-			return NewInvalidSignatureError("Field optional.timestamp is not is not an integer")
+			return NewInvalidSignatureError("Field optional.timestamp is not an integer")
 		}
 		s.untrustedTimestamp = &intTimestamp
 	}
diff --git a/signature/simple.go b/signature/simple.go
index 30df997d86..94a8465930 100644
--- a/signature/simple.go
+++ b/signature/simple.go
@@ -105,13 +105,7 @@ var _ json.Unmarshaler = (*untrustedSignature)(nil)
 
 // UnmarshalJSON implements the json.Unmarshaler interface
 func (s *untrustedSignature) UnmarshalJSON(data []byte) error {
-	err := s.strictUnmarshalJSON(data)
-	if err != nil {
-		if formatErr, ok := err.(internal.JSONFormatError); ok {
-			err = internal.NewInvalidSignatureError(formatErr.Error())
-		}
-	}
-	return err
+	return internal.JSONFormatToInvalidSignatureError(s.strictUnmarshalJSON(data))
 }
 
 // strictUnmarshalJSON is UnmarshalJSON, except that it may return the internal.JSONFormatError error type.
@@ -149,7 +143,7 @@ func (s *untrustedSignature) strictUnmarshalJSON(data []byte) error {
 	if gotTimestamp {
 		intTimestamp := int64(timestamp)
 		if float64(intTimestamp) != timestamp {
-			return internal.NewInvalidSignatureError("Field optional.timestamp is not is not an integer")
+			return internal.NewInvalidSignatureError("Field optional.timestamp is not an integer")
 		}
 		s.untrustedTimestamp = &intTimestamp
 	}