diff --git a/WHATSNEW b/WHATSNEW
index 93d3909c7..f5c34f134 100644
--- a/WHATSNEW
+++ b/WHATSNEW
@@ -154,6 +154,9 @@ Fixed bugs:
 * War task failed with "No WEB-INF/web.xml file was added" when called
   a second time. Bugzilla 43121.
 
+* FilterMapper could throw an NPE.
+  Bugzilla 43292.
+
 Other changes:
 --------------
 * <script> now has basic support for JavaFX scripts
diff --git a/src/main/org/apache/tools/ant/util/FileUtils.java b/src/main/org/apache/tools/ant/util/FileUtils.java
index dc3252cd3..95e463972 100644
--- a/src/main/org/apache/tools/ant/util/FileUtils.java
+++ b/src/main/org/apache/tools/ant/util/FileUtils.java
@@ -930,7 +930,7 @@ public class FileUtils {
                 textBuffer.append(new String(buffer, 0, bufferLength));
             }
         }
-        return (textBuffer == null) ? null : textBuffer.toString();
+        return (textBuffer == null) ? "" : textBuffer.toString();
     }
 
     /**