diff --git a/internal/handlers/handlers_internal_test.go b/internal/handlers/handlers_internal_test.go
index bfcb01a..1ee7fc4 100644
--- a/internal/handlers/handlers_internal_test.go
+++ b/internal/handlers/handlers_internal_test.go
@@ -4,6 +4,7 @@ import (
 	"bytes"
 	"context"
 	"database/sql"
+	"encoding/json"
 	"image"
 	"image/color"
 	"image/jpeg"
@@ -291,3 +292,54 @@ func TestHandleImage_InvalidFitMode_Returns400(t *testing.T) {
 		t.Fatalf("status = %d, want %d", status, http.StatusBadRequest)
 	}
 }
+
+// TestHandleImage_InvalidQuality_Returns400 verifies that the plain image
+// route answers a q that is not a whole number from 1 to 100 with 400 naming
+// q and the value, instead of serving the image at the default quality 85.
+func TestHandleImage_InvalidQuality_Returns400(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		q, wantError string
+	}{
+		{"banana", `invalid q: not a number, got "banana"`},
+		{"0", `invalid q: must be from 1 to 100, got "0"`},
+		{"101", `invalid q: must be from 1 to 100, got "101"`},
+	}
+
+	for _, tt := range tests {
+		t.Run("q="+tt.q, func(t *testing.T) {
+			t.Parallel()
+
+			fix := setupTestHandler(t)
+
+			r := chi.NewRouter()
+			r.Get("/v1/image/*", fix.handler.HandleImage())
+
+			req := httptest.NewRequestWithContext(t.Context(), http.MethodGet,
+				"/v1/image/"+fix.goodHost+"/images/photo.jpg/50x50.jpeg?q="+tt.q, nil)
+			rec := httptest.NewRecorder()
+
+			r.ServeHTTP(rec, req)
+
+			if rec.Code != http.StatusBadRequest {
+				t.Fatalf("status = %d, want %d", rec.Code, http.StatusBadRequest)
+			}
+
+			t.Logf("GET %s: %d %s", req.URL, rec.Code, rec.Body)
+
+			var body struct {
+				Error string `json:"error"`
+			}
+
+			err := json.NewDecoder(rec.Body).Decode(&body)
+			if err != nil {
+				t.Fatalf("decoding response body: %v", err)
+			}
+
+			if body.Error != tt.wantError {
+				t.Errorf("error = %q, want %q", body.Error, tt.wantError)
+			}
+		})
+	}
+}