diff --git a/prism/src/param/BigRational.java b/prism/src/param/BigRational.java
index 367b6d5a..4a0f384f 100644
--- a/prism/src/param/BigRational.java
+++ b/prism/src/param/BigRational.java
@@ -474,6 +474,7 @@ public final class BigRational implements Comparable<BigRational>
 			num = this.num.pow(exponent);
 			den = this.den.pow(exponent);
 		} else { // exponent < 0
+			exponent = -exponent;
 			num = this.den.pow(exponent);
 			den = this.num.pow(exponent);
 		}