diff --git a/logback-core/src/main/java/ch/qos/logback/core/joran/spi/InterpretationContext.java b/logback-core/src/main/java/ch/qos/logback/core/joran/spi/InterpretationContext.java
index e9e6d95483..2c0292b5ff 100644
--- a/logback-core/src/main/java/ch/qos/logback/core/joran/spi/InterpretationContext.java
+++ b/logback-core/src/main/java/ch/qos/logback/core/joran/spi/InterpretationContext.java
@@ -132,8 +132,10 @@ public void addSubstitutionProperties(Properties props) {
     if (props == null) {
       return;
     }
-    for (String key : props.stringPropertyNames()) {
-      addSubstitutionProperty(key, props.getProperty(key));
+    for(Object keyObject: props.keySet()) {
+      String key = (String) keyObject;
+      String val = props.getProperty(key);
+      addSubstitutionProperty(key, val);
     }
   }