diff --git a/implementation/src/main/java/io/smallrye/config/SmallRyeConfigBuilder.java b/implementation/src/main/java/io/smallrye/config/SmallRyeConfigBuilder.java
index 3dea990b0..e4f30ad65 100644
--- a/implementation/src/main/java/io/smallrye/config/SmallRyeConfigBuilder.java
+++ b/implementation/src/main/java/io/smallrye/config/SmallRyeConfigBuilder.java
@@ -697,6 +697,9 @@ public SmallRyeConfig build() {
                 .forEach(customizer -> customizer.configBuilder(SmallRyeConfigBuilder.this));
 
         ConfigMappingProvider mappingProvider = mappingsBuilder.build();
+        for (Map.Entry<String, String> entry : mappingProvider.getDefaultValues().entrySet()) {
+            defaultValues.putIfAbsent(entry.getKey(), entry.getValue());
+        }
         SmallRyeConfig config = new SmallRyeConfig(this);
         ConfigMappings.mapConfiguration(config, mappingProvider);
         return config;
diff --git a/implementation/src/test/java/io/smallrye/config/ConfigSourceFactoryTest.java b/implementation/src/test/java/io/smallrye/config/ConfigSourceFactoryTest.java
index 46e7fd2ef..8eb0b96df 100644
--- a/implementation/src/test/java/io/smallrye/config/ConfigSourceFactoryTest.java
+++ b/implementation/src/test/java/io/smallrye/config/ConfigSourceFactoryTest.java
@@ -48,10 +48,11 @@ void expression() {
         SmallRyeConfig config = new SmallRyeConfigBuilder()
                 .addDefaultInterceptors()
                 .withSources(new ExpressionConfigSourceFactory())
-                .withSources(config("expression.value", "12${DEFAULT:}"))
-                .withSources(new EnvConfigSource(Map.of("DEFAULT", "34"), 100))
+                .withSources(new EnvConfigSource(Map.of("DEFAULT", "1234"), 100))
+                .withMapping(Expression.class)
                 .build();
 
+        assertEquals("1234", config.getConfigMapping(Expression.class).value());
         assertEquals("1234", config.getRawValue("factory.expression"));
     }