diff --git a/frontend/src/components/TiffImageViewer.tsx b/frontend/src/components/TiffImageViewer.tsx
index 8160daee..e84b3af4 100644
--- a/frontend/src/components/TiffImageViewer.tsx
+++ b/frontend/src/components/TiffImageViewer.tsx
@@ -133,7 +133,7 @@ function TiffImageViewer({ url, variant, height = 460 }: MyProps) {
 
         if (variant === "value") {
             const stops = CMAPS[cmapName] ?? CMAPS.Viridis;
-            const span = hi - lo || 1;
+            const span = hi - lo > 0 ? hi - lo : 1;  // guard zero/negative (e.g. Min>Max or a cleared field)
             for (let i = 0; i < data.length; i++) {
                 const v = data[i];
                 const j = i * 4;