diff --git a/prism/src/explicit/ProbModelChecker.java b/prism/src/explicit/ProbModelChecker.java
index fd70ca4d..61546902 100644
--- a/prism/src/explicit/ProbModelChecker.java
+++ b/prism/src/explicit/ProbModelChecker.java
@@ -1026,7 +1026,7 @@ public class ProbModelChecker extends NonProbModelChecker
 		else if (Expression.isCoSafeLTLSyntactic(expr)) {
 			return checkRewardCoSafeLTL(model, modelRewards, expr, minMax, statesOfInterest);
 		}
-		throw new PrismException("Invalid contents for an R operator: " + expr);
+		throw new PrismException("R operator contains a path formula that is not syntactically co-safe: " + expr);
 	}
 	
 	/**
diff --git a/prism/src/prism/NondetModelChecker.java b/prism/src/prism/NondetModelChecker.java
index ca9d4b38..4865b306 100644
--- a/prism/src/prism/NondetModelChecker.java
+++ b/prism/src/prism/NondetModelChecker.java
@@ -1189,7 +1189,7 @@ public class NondetModelChecker extends NonProbModelChecker
 		else if (Expression.isCoSafeLTLSyntactic(expr)) {
 			return checkRewardCoSafeLTL(expr, stateRewards, transRewards, min);
 		}
-		throw new PrismException("Invalid contents for an R operator: " + expr);
+		throw new PrismException("R operator contains a path formula that is not syntactically co-safe: " + expr);
 	}
 	
 	// reach reward
diff --git a/prism/src/prism/ProbModelChecker.java b/prism/src/prism/ProbModelChecker.java
index 40bca11a..2cc5221a 100644
--- a/prism/src/prism/ProbModelChecker.java
+++ b/prism/src/prism/ProbModelChecker.java
@@ -863,7 +863,7 @@ public class ProbModelChecker extends NonProbModelChecker
 		else if (Expression.isCoSafeLTLSyntactic(expr)) {
 			throw new PrismNotSupportedException("Co-safe reward properties not yet supported for DTMCs in this engine");
 		}
-		throw new PrismException("Invalid contents for an R operator: " + expr);
+		throw new PrismException("R operator contains a path formula that is not syntactically co-safe: " + expr);
 	}
 	
 	// reach reward