diff --git a/nb-apis/adapters-api/src/main/java/io/nosqlbench/adapters/api/templating/StrInterpolator.java b/nb-apis/adapters-api/src/main/java/io/nosqlbench/adapters/api/templating/StrInterpolator.java
index 08271ada2..f78de1a25 100644
--- a/nb-apis/adapters-api/src/main/java/io/nosqlbench/adapters/api/templating/StrInterpolator.java
+++ b/nb-apis/adapters-api/src/main/java/io/nosqlbench/adapters/api/templating/StrInterpolator.java
@@ -168,10 +168,11 @@ public class StrInterpolator implements Function<String, String> {
 
     public static class MultiMap extends StrLookup<String> {
 
-        private final List<Map<String, ?>> maps = new ArrayList<>();
         private final String warnPrefix = "UNSET";
         private final Map<String,String> accesses = new LinkedHashMap<>();
         private final Map<String,String> extractedDefaults = new LinkedHashMap<>();
+        private final Map<String,String> overrides = new LinkedHashMap<>();
+        private final List<Map<String, ?>> maps = new ArrayList<>();
 
         public void add(Map<String, ?> addedMap) {
             maps.add(addedMap);
@@ -179,25 +180,57 @@ public class StrInterpolator implements Function<String, String> {
 
         @Override
         public String lookup(String key) {
+            //String original = key;
             String value = null;
+            char substitution = ' ';
 
             String[] parts = key.split("[:,]", 2);
             if (parts.length == 2) {
                 key = parts[0];
                 value = parts[1];
+                if ( value.length() > 0 ) {
+                    substitution = value.charAt(0);
+                } else {
+                    substitution = ' ';
+                }
+                if ( substitution == '-' || substitution == '=' || substitution == '?' || substitution == '+') {
+                    if ( value.length() < 2 ) {
+                        value = null;
+                    } else {
+                        value = value.substring(1);
+                    }
+                } else {
+                    substitution = ' ';
+                }
                 if (!extractedDefaults.containsKey(key)) {
-                    extractedDefaults.put(key,value);
+                    extractedDefaults.put(key, value);
+                }
+                if (!overrides.containsKey(key)) {
+                    if ( substitution == '=' ) {
+                        overrides.put(key, value);
+                        //System.out.println(key+"="+value);
+                    } else if ( substitution == '?' ) {
+                        throw new NullPointerException("Parameter "+key+" is not set");
+                    }
                 }
             }
 
-            for (Map<String, ?> map : maps) {
-                Object val = map.get(key);
+            if ( substitution != '+' ) {
+                Object val = overrides.get(key);
                 if (val != null) {
                     value = val.toString();
-                    break;
+                    //System.out.println("for: '"+original+"': "+key+"->"+value);
+                } else {
+                    for (Map<String, ?> map : maps) {
+                        val = map.get(key);
+                        if (val != null) {
+                            value = val.toString();
+                            break;
+                        }
+                    }
                 }
+                value = (value==null? extractedDefaults.get(key) : value);
             }
-            value = (value==null? extractedDefaults.get(key) : value);
 
             value = (value != null) ? value : warnPrefix + ":" + key;
 
diff --git a/nb-apis/adapters-api/src/test/java/io/nosqlbench/adapters/api/templating/StrInterpolatorTest.java b/nb-apis/adapters-api/src/test/java/io/nosqlbench/adapters/api/templating/StrInterpolatorTest.java
index 0e59edbc6..17cf95ce8 100644
--- a/nb-apis/adapters-api/src/test/java/io/nosqlbench/adapters/api/templating/StrInterpolatorTest.java
+++ b/nb-apis/adapters-api/src/test/java/io/nosqlbench/adapters/api/templating/StrInterpolatorTest.java
@@ -16,6 +16,8 @@
 
 package io.nosqlbench.adapters.api.templating;
 
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
 import org.junit.jupiter.api.Test;
 
 import java.util.ArrayList;
@@ -115,6 +117,30 @@ public class StrInterpolatorTest {
         assertThat(a).isEqualTo("START\n# TEMPLATE(blahblah,blah)\nUniform(0,1000000000)->int;");
     }
 
+    @Test
+    public void sustitutionTests() {
+        StrInterpolator interp = new StrInterpolator(abcd);
+        String a = interp.apply("-${setkey:=setme}-${setkey}-");
+        assertThat(a).isEqualTo("-setme-setme-");
+        String b = interp.apply("-${setkey2:-setme}-${setkey2:-setyou}-");
+        assertThat(b).isEqualTo("-setme-setyou-");
+        String c = interp.apply("-${setkey:=setme}-${setkey3:+setme}-${setkey:+setyou}-");
+        assertThat(c).isEqualTo("-setme-setme-setyou-");
+    }
+
+    @Test
+    public void shouldThrowException() {
+        StrInterpolator interp = new StrInterpolator(abcd);
+
+        // Test for missing variable or invalid syntax
+        Exception exception = assertThrows(NullPointerException.class, () -> {
+            interp.apply("-${unsetKey:?unset exception}-");
+        });
+
+        // Verify the exception message if necessary
+        assertThat(exception.getMessage()).contains("unsetKey");
+    }
+
     @Test
     public void shouldReturnWarningWhenUnmatched() {
         StrInterpolator interp = new StrInterpolator(abcd);