diff --git a/components/content/PdockqMatrix.vue b/components/content/PdockqMatrix.vue
index 045526907551078c966ab917855a76f80c9f6f86..4b0c33505a1e8a4b640c912209a24ce34cdbb861 100644
--- a/components/content/PdockqMatrix.vue
+++ b/components/content/PdockqMatrix.vue
@@ -169,8 +169,8 @@ function on(mark) {
         r.on("mouseover", function (event) {
             const elem = d3.select(event.srcElement)
             const fillColor = elem.attr("fill")
-            const elemFill = d3.color(fillColor).darker()
-            elem.attr("stroke-width", 4).attr("stroke", elemFill.toString()).attr("cursor", "pointer")
+            const elemFill = d3.color(fillColor)?.darker()
+            if (elemFill) elem.attr("stroke-width", 4).attr("stroke", elemFill.toString()).attr("cursor", "pointer")
         })
         r.on("mouseout", function (event) {
             d3.select(event.srcElement).attr("stroke-width", null).attr("stroke", null).attr("cursor", "unset")