diff --git a/WHATSNEW b/WHATSNEW
index fd9645999..3b843092c 100644
--- a/WHATSNEW
+++ b/WHATSNEW
@@ -7,6 +7,10 @@ Changes that could break older environments:
 Fixed bugs:
 -----------
 
+ * External XML catalog resolver failed to use project basedir when given an
+   unmentioned relative path like the internal resolver does.
+   Bugzilla Report 52754.
+
 Other changes:
 --------------
 
diff --git a/src/main/org/apache/tools/ant/types/XMLCatalog.java b/src/main/org/apache/tools/ant/types/XMLCatalog.java
index 77a90d11e..1edc10011 100644
--- a/src/main/org/apache/tools/ant/types/XMLCatalog.java
+++ b/src/main/org/apache/tools/ant/types/XMLCatalog.java
@@ -1072,6 +1072,13 @@ public class XMLCatalog extends DataType
                 // Apache resolver's resolveEntity method to cover
                 // this possibility.
                 //
+                if (base == null) {
+                    try {
+                        base = FILE_UTILS.getFileURL(getProject().getBaseDir()).toString();
+                    } catch (MalformedURLException x) {
+                        throw new TransformerException(x);
+                    }
+                }
                 try {
                     result =
                         (SAXSource) resolve.invoke(resolverImpl,