diff --git a/log4j-api/src/main/java/org/apache/logging/log4j/message/ParameterFormatter.java b/log4j-api/src/main/java/org/apache/logging/log4j/message/ParameterFormatter.java
index 79c447a..d827c7c 100644
--- a/log4j-api/src/main/java/org/apache/logging/log4j/message/ParameterFormatter.java
+++ b/log4j-api/src/main/java/org/apache/logging/log4j/message/ParameterFormatter.java
@@ -566,20 +566,22 @@
             final Map<?, ?> oMap = (Map<?, ?>) o;
             str.append('{');
             boolean isFirst = true;
-            for (final Object o1 : oMap.entrySet()) {
-                final Map.Entry<?, ?> current = (Map.Entry<?, ?>) o1;
-                if (isFirst) {
-                    isFirst = false;
-                } else {
-                    str.append(", ");
+            synchronized (oMap) {
+                for (final Object o1 : oMap.entrySet()) {
+                    final Map.Entry<?, ?> current = (Map.Entry<?, ?>) o1;
+                    if (isFirst) {
+                        isFirst = false;
+                    } else {
+                        str.append(", ");
+                    }
+                    final Object key = current.getKey();
+                    final Object value = current.getValue();
+                    recursiveDeepToString(key, str, new HashSet<>(dejaVu));
+                    str.append('=');
+                    recursiveDeepToString(value, str, new HashSet<>(dejaVu));
                 }
-                final Object key = current.getKey();
-                final Object value = current.getValue();
-                recursiveDeepToString(key, str, new HashSet<>(dejaVu));
-                str.append('=');
-                recursiveDeepToString(value, str, new HashSet<>(dejaVu));
+                str.append('}');
             }
-            str.append('}');
         }
     }
 
@@ -596,16 +598,17 @@
             final Collection<?> oCol = (Collection<?>) o;
             str.append('[');
             boolean isFirst = true;
-            for (final Object anOCol : oCol) {
-                if (isFirst) {
-                    isFirst = false;
-                } else {
-                    str.append(", ");
+            synchronized (oCol) {
+                for (final Object anOCol : oCol) {
+                    if (isFirst) {
+                        isFirst = false;
+                    } else {
+                        str.append(", ");
+                    }
+                    recursiveDeepToString(anOCol, str, new HashSet<>(dejaVu));
                 }
-                recursiveDeepToString(anOCol, str, new HashSet<>(dejaVu));
-            }
-            str.append(']');
-        }
+                str.append(']');
+            }}
     }
 
     private static void tryObjectToString(final Object o, final StringBuilder str) {