diff --git a/client/client.go b/client/client.go
index 29948fb..529ce2d 100644
--- a/client/client.go
+++ b/client/client.go
@@ -206,8 +206,11 @@ func (c *Client) do(ctx context.Context, method, path string, addr stdaddr.Addre
 		}
 
 		// Try to unmarshal the response body to a known vspd error.
+		d := json.NewDecoder(bytes.NewReader(respBody))
+		d.DisallowUnknownFields()
+
 		var apiError types.ErrorResponse
-		err = json.Unmarshal(respBody, &apiError)
+		err = d.Decode(&apiError)
 		if err == nil {
 			return apiError
 		}
diff --git a/client/client_test.go b/client/client_test.go
new file mode 100644
index 0000000..8543caa
--- /dev/null
+++ b/client/client_test.go
@@ -0,0 +1,102 @@
+package client
+
+import (
+	"context"
+	"errors"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"github.com/decred/dcrd/txscript/v4/stdaddr"
+	"github.com/decred/slog"
+	"github.com/decred/vspd/types"
+)
+
+func NoopSign(ctx context.Context, message string, address stdaddr.Address) ([]byte, error) {
+	return nil, nil
+}
+
+func NoopValidate(resp *http.Response, body []byte, serverPubkey []byte) error {
+	return nil
+}
+
+// TestErrorDetails ensures errors returned by client.do contain adequate
+// information for debugging (HTTP status and response body).
+func TestErrorDetails(t *testing.T) {
+
+	tests := map[string]struct {
+		httpStatus        int
+		responseBodyBytes []byte
+		expectedErr       string
+		vspdError         bool
+	}{
+		"500, vspd error": {
+			httpStatus:        500,
+			responseBodyBytes: []byte(`{"code": 1, "message": "bad request"}`),
+			expectedErr:       `bad request`,
+			vspdError:         true,
+		},
+		"500, no body": {
+			httpStatus:        500,
+			responseBodyBytes: nil,
+			expectedErr:       `http status 500 (Internal Server Error) with no body`,
+			vspdError:         false,
+		},
+		"500, non vspd error": {
+			httpStatus:        500,
+			responseBodyBytes: []byte(`an error occurred`),
+			expectedErr:       `http status 500 (Internal Server Error) with body "an error occurred"`,
+			vspdError:         false,
+		},
+		"500, non vspd error (json)": {
+			httpStatus:        500,
+			responseBodyBytes: []byte(`{"some": "json"}`),
+			expectedErr:       `http status 500 (Internal Server Error) with body "{\"some\": \"json\"}"`,
+			vspdError:         false,
+		},
+	}
+
+	for testName, testData := range tests {
+		t.Run(testName, func(t *testing.T) {
+
+			testServer := httptest.NewServer(http.HandlerFunc(func(res http.ResponseWriter, req *http.Request) {
+				res.WriteHeader(testData.httpStatus)
+				_, err := res.Write(testData.responseBodyBytes)
+				if err != nil {
+					t.Fatalf("writing response body failed: %v", err)
+				}
+			}))
+
+			client := Client{
+				URL:      testServer.URL,
+				PubKey:   []byte("fake pubkey"),
+				Sign:     NoopSign,
+				Validate: NoopValidate,
+				Log:      slog.Disabled,
+			}
+
+			var resp interface{}
+			err := client.do(context.TODO(), http.MethodGet, "", nil, &resp, nil)
+
+			testServer.Close()
+
+			if err == nil {
+				t.Fatalf("client.do did not return an error")
+			}
+
+			if err.Error() != testData.expectedErr {
+				t.Fatalf("client.do returned incorrect error, expected %q, got %q",
+					testData.expectedErr, err.Error())
+			}
+
+			if testData.vspdError {
+				// Error should be unwrappable as a vspd error response.
+				var e types.ErrorResponse
+				if !errors.As(err, &e) {
+					t.Fatal("unable to unwrap vspd error")
+				}
+			}
+
+		})
+	}
+}