diff --git a/core/src/main/java/hudson/model/RunMap.java b/core/src/main/java/hudson/model/RunMap.java
index f5897bb5e8..ad2ae988c6 100644
--- a/core/src/main/java/hudson/model/RunMap.java
+++ b/core/src/main/java/hudson/model/RunMap.java
@@ -186,7 +186,9 @@ public final class RunMap<R extends Run<?,R>> extends AbstractLazyLoadRunMap<R>
         if (rootDir.isDirectory()) {
             throw new IllegalStateException(rootDir + " already existed; will not overwite with " + r);
         }
-        proposeNewNumber(r.getNumber());
+        if (!r.getClass().getName().equals("hudson.matrix.MatrixRun")) { // JENKINS-26739: grandfathered in
+            proposeNewNumber(r.getNumber());
+        }
         rootDir.mkdirs();
         return super._put(r);
     }