diff --git a/src/libexpr/eval-cache.cc b/src/libexpr/eval-cache.cc
index 2fc69e79674..1538eb05679 100644
--- a/src/libexpr/eval-cache.cc
+++ b/src/libexpr/eval-cache.cc
@@ -581,7 +581,7 @@ std::string AttrCursor::getString()
     auto & v = forceValue();
 
     if (v.type() != nString && v.type() != nPath)
-        root->state.error<TypeError>("'%s' is not a string but %s", getAttrPathStr()).debugThrow();
+        root->state.error<TypeError>("'%s' is not a string but %s", getAttrPathStr(), showType(v)).debugThrow();
 
     return v.type() == nString ? v.c_str() : v.path().to_string();
 }
@@ -630,7 +630,7 @@ string_t AttrCursor::getStringWithContext()
     else if (v.type() == nPath)
         return {v.path().to_string(), {}};
     else
-        root->state.error<TypeError>("'%s' is not a string but %s", getAttrPathStr()).debugThrow();
+        root->state.error<TypeError>("'%s' is not a string but %s", getAttrPathStr(), showType(v)).debugThrow();
 }
 
 bool AttrCursor::getBool()