diff --git a/liquidjava-example/src/main/java/testSuite/ErrorUnknownMethodInOperation.java b/liquidjava-example/src/main/java/testSuite/ErrorUnknownMethodInOperation.java
index 177dab74..3ca462a4 100644
--- a/liquidjava-example/src/main/java/testSuite/ErrorUnknownMethodInOperation.java
+++ b/liquidjava-example/src/main/java/testSuite/ErrorUnknownMethodInOperation.java
@@ -9,4 +9,17 @@ public void lengthPlusOne(String s) {
         @Refinement("_ > 0")
         int x = s.length() + 1; // Expect: Refinement Error
     }
+
+    static void take(@Refinement("_ > 0") int amount) {}
+
+    static void takeTrue(@Refinement("_ == true") boolean flag) {}
+
+    // also when passed straight as an argument (issue #451)
+    public void lengthAsArgument(String s) {
+        take(s.length()); // Expect: Refinement Error
+    }
+
+    public void isEmptyAsArgument(String s) {
+        takeTrue(s.isEmpty()); // Expect: Refinement Error
+    }
 }
diff --git a/liquidjava-verifier/src/main/java/liquidjava/processor/refinement_checker/general_checkers/MethodsFunctionsChecker.java b/liquidjava-verifier/src/main/java/liquidjava/processor/refinement_checker/general_checkers/MethodsFunctionsChecker.java
index 8c8f4ade..0e21f2fb 100644
--- a/liquidjava-verifier/src/main/java/liquidjava/processor/refinement_checker/general_checkers/MethodsFunctionsChecker.java
+++ b/liquidjava-verifier/src/main/java/liquidjava/processor/refinement_checker/general_checkers/MethodsFunctionsChecker.java
@@ -16,6 +16,7 @@
 import liquidjava.processor.refinement_checker.object_checkers.AuxHierarchyRefinementsPassage;
 import liquidjava.processor.refinement_checker.object_checkers.AuxStateHandler;
 import liquidjava.rj_language.Predicate;
+import liquidjava.rj_language.ast.LiteralBoolean;
 import liquidjava.rj_language.ast.formatter.ArgumentDisplayNames;
 import liquidjava.utils.GeneratedNames;
 import liquidjava.utils.Utils;
@@ -456,7 +457,10 @@ private String createVariableRepresentingArgument(CtExpression<?> iArg, Variable
         Predicate met = (Predicate) iArg.getMetadata(Keys.REFINEMENT);
         if (met == null)
             met = new Predicate();
-        if (!met.getVariableNames().contains(Keys.WILDCARD))
+        // a bare `true` means nothing is known about the argument (e.g. the result of an unrefined call), not that it
+        // equals true
+        boolean unknown = met.getExpression()instanceof LiteralBoolean b && b.isBooleanTrue();
+        if (!unknown && !met.getVariableNames().contains(Keys.WILDCARD))
             met = Predicate.createEquals(Predicate.createVar(Keys.WILDCARD), met);
         String nVar = String.format(Formats.INSTANCE, fArg.getName(), rtc.getContext().getCounter());
         rtc.getContext().addInstanceToContext(nVar, fArg.getType(), met.substituteVariable(Keys.WILDCARD, nVar), iArg);