diff --git a/api/read/read.go b/api/read/read.go
index 2f7348bd..9c5ebd07 100644
--- a/api/read/read.go
+++ b/api/read/read.go
@@ -6,6 +6,7 @@ import (
 	"errors"
 	"io"
 	"net/http"
+	"strings"
 
 	"google.golang.org/protobuf/encoding/protojson"
 	"google.golang.org/protobuf/proto"
@@ -33,7 +34,9 @@ func ProtoJSON(r io.Reader, m proto.Message) error {
 
 	switch err := protojson.Unmarshal(data, m); {
 	case errors.Is(err, proto.Error):
-		return badProtoJSONError(err.Error())
+		// trim the proto prefix for the message
+		s := strings.TrimSpace(strings.TrimPrefix(err.Error(), "proto:"))
+		return badProtoJSONError(s)
 	default:
 		return err
 	}
diff --git a/authority/admin/api/policy_test.go b/authority/admin/api/policy_test.go
index fffa84f7..d0c97729 100644
--- a/authority/admin/api/policy_test.go
+++ b/authority/admin/api/policy_test.go
@@ -376,7 +376,7 @@ func TestPolicyAdminResponder_CreateAuthorityPolicy(t *testing.T) {
 				// a syntax error (in the tests). If the message doesn't start with "proto",
 				// we expect a full string match.
 				if strings.HasPrefix(tc.err.Message, "proto:") {
-					assert.True(t, strings.Contains(tc.err.Message, "syntax error"))
+					assert.True(t, strings.Contains(ae.Message, "syntax error"))
 				} else {
 					assert.Equals(t, tc.err.Message, ae.Message)
 				}
@@ -634,7 +634,7 @@ func TestPolicyAdminResponder_UpdateAuthorityPolicy(t *testing.T) {
 				// a syntax error (in the tests). If the message doesn't start with "proto",
 				// we expect a full string match.
 				if strings.HasPrefix(tc.err.Message, "proto:") {
-					assert.True(t, strings.Contains(tc.err.Message, "syntax error"))
+					assert.True(t, strings.Contains(ae.Message, "syntax error"))
 				} else {
 					assert.Equals(t, tc.err.Message, ae.Message)
 				}
@@ -1081,7 +1081,7 @@ func TestPolicyAdminResponder_CreateProvisionerPolicy(t *testing.T) {
 				// a syntax error (in the tests). If the message doesn't start with "proto",
 				// we expect a full string match.
 				if strings.HasPrefix(tc.err.Message, "proto:") {
-					assert.True(t, strings.Contains(tc.err.Message, "syntax error"))
+					assert.True(t, strings.Contains(ae.Message, "syntax error"))
 				} else {
 					assert.Equals(t, tc.err.Message, ae.Message)
 				}
@@ -1292,7 +1292,7 @@ func TestPolicyAdminResponder_UpdateProvisionerPolicy(t *testing.T) {
 				// a syntax error (in the tests). If the message doesn't start with "proto",
 				// we expect a full string match.
 				if strings.HasPrefix(tc.err.Message, "proto:") {
-					assert.True(t, strings.Contains(tc.err.Message, "syntax error"))
+					assert.True(t, strings.Contains(ae.Message, "syntax error"))
 				} else {
 					assert.Equals(t, tc.err.Message, ae.Message)
 				}
@@ -1694,7 +1694,7 @@ func TestPolicyAdminResponder_CreateACMEAccountPolicy(t *testing.T) {
 				// a syntax error (in the tests). If the message doesn't start with "proto",
 				// we expect a full string match.
 				if strings.HasPrefix(tc.err.Message, "proto:") {
-					assert.True(t, strings.Contains(tc.err.Message, "syntax error"))
+					assert.True(t, strings.Contains(ae.Message, "syntax error"))
 				} else {
 					assert.Equals(t, tc.err.Message, ae.Message)
 				}
@@ -1880,7 +1880,7 @@ func TestPolicyAdminResponder_UpdateACMEAccountPolicy(t *testing.T) {
 				// a syntax error (in the tests). If the message doesn't start with "proto",
 				// we expect a full string match.
 				if strings.HasPrefix(tc.err.Message, "proto:") {
-					assert.True(t, strings.Contains(tc.err.Message, "syntax error"))
+					assert.True(t, strings.Contains(ae.Message, "syntax error"))
 				} else {
 					assert.Equals(t, tc.err.Message, ae.Message)
 				}
diff --git a/authority/admin/api/provisioner_test.go b/authority/admin/api/provisioner_test.go
index de7c3646..486b6cda 100644
--- a/authority/admin/api/provisioner_test.go
+++ b/authority/admin/api/provisioner_test.go
@@ -430,7 +430,7 @@ func TestHandler_CreateProvisioner(t *testing.T) {
 				assert.Equals(t, []string{"application/json"}, res.Header["Content-Type"])
 
 				if strings.HasPrefix(tc.err.Message, "proto:") {
-					assert.True(t, strings.Contains(tc.err.Message, "syntax error"))
+					assert.True(t, strings.Contains(adminErr.Message, "syntax error"))
 				} else {
 					assert.Equals(t, tc.err.Message, adminErr.Message)
 				}
@@ -1087,7 +1087,7 @@ func TestHandler_UpdateProvisioner(t *testing.T) {
 				assert.Equals(t, []string{"application/json"}, res.Header["Content-Type"])
 
 				if strings.HasPrefix(tc.err.Message, "proto:") {
-					assert.True(t, strings.Contains(tc.err.Message, "syntax error"))
+					assert.True(t, strings.Contains(adminErr.Message, "syntax error"))
 				} else {
 					assert.Equals(t, tc.err.Message, adminErr.Message)
 				}