diff --git a/hwe/ts/util/RandUtil.ts b/hwe/ts/util/RandUtil.ts
index 8a778fd4..ef3d4908 100644
--- a/hwe/ts/util/RandUtil.ts
+++ b/hwe/ts/util/RandUtil.ts
@@ -35,6 +35,9 @@ export class RandUtil {
         if (prob === 0.5){
             return this.nextBit();
         }
+        if (prob <= 0){
+            return false;
+        }
         return this.nextFloat1() < prob;
     }
 
diff --git a/src/sammo/RandUtil.php b/src/sammo/RandUtil.php
index 962fd5c4..591214d2 100644
--- a/src/sammo/RandUtil.php
+++ b/src/sammo/RandUtil.php
@@ -42,9 +42,12 @@ class RandUtil
         if ($prob >= 1) {
             return true;
         }
-        if($prob === 0.5){
+        if ($prob === 0.5){
             return $this->nextBit();
         }
+        if ($prob <= 0){
+            return false;
+        }
         return $this->nextFloat1() < $prob;
     }