diff --git a/data-prepper-expression/src/main/java/org/opensearch/dataprepper/expression/JoinExpressionFunction.java b/data-prepper-expression/src/main/java/org/opensearch/dataprepper/expression/JoinExpressionFunction.java
index 6ca35782af..d52729757b 100644
--- a/data-prepper-expression/src/main/java/org/opensearch/dataprepper/expression/JoinExpressionFunction.java
+++ b/data-prepper-expression/src/main/java/org/opensearch/dataprepper/expression/JoinExpressionFunction.java
@@ -54,15 +54,15 @@ public Object evaluate(final List<Object> args, Event event, Function<Object, Ob
         }
 
         try {
-            final List<Object> sourceList = event.get(sourceKey, List.class);
-            return joinList(sourceList, delimiter);
-        } catch (Exception e) {
-            try {
+            if (event.isValueAList(sourceKey)) {
+                final List<Object> sourceList = event.get(sourceKey, List.class);
+                return joinList(sourceList, delimiter);
+            } else {
                 final Map<String, Object> sourceMap = event.get(sourceKey, Map.class);
                 return joinListsInMap(sourceMap, delimiter);
-            } catch (Exception ex) {
-                throw new RuntimeException("Unable to perform join function on " + sourceKey, ex);
             }
+        } catch (Exception ex) {
+            throw new RuntimeException("Unable to perform join function on " + sourceKey, ex);
         }
     }
 
diff --git a/data-prepper-expression/src/test/java/org/opensearch/dataprepper/expression/JoinExpressionFunctionTest.java b/data-prepper-expression/src/test/java/org/opensearch/dataprepper/expression/JoinExpressionFunctionTest.java
index 48da627e5e..3ec60a73c5 100644
--- a/data-prepper-expression/src/test/java/org/opensearch/dataprepper/expression/JoinExpressionFunctionTest.java
+++ b/data-prepper-expression/src/test/java/org/opensearch/dataprepper/expression/JoinExpressionFunctionTest.java
@@ -94,6 +94,13 @@ void testSourceFieldNotExistsInEventThrowsException() {
                 () -> joinExpressionFunction.evaluate(List.of("/missingKey"), testEvent, testFunction));
     }
 
+    @Test
+    void testSourceFieldNotListOrMapThrowsException() {
+        testEvent = createTestEvent(Map.of("key", "value"));
+        assertThrows(RuntimeException.class,
+                () -> joinExpressionFunction.evaluate(List.of("/key"), testEvent, testFunction));
+    }
+
     private static Stream<Arguments> joinSingleList() {
         final String inputData = "{\"list\":[\"string\", 1, true]}";
         return Stream.of(