diff --git a/threadlib.scad b/threadlib.scad
index 8f9e2ea..132efb5 100644
--- a/threadlib.scad
+++ b/threadlib.scad
@@ -17,9 +17,12 @@ include <THREAD_TABLE.scad>
 function thread_specs(designator, table=THREAD_TABLE) =
     /* Returns thread specs of thread-type 'designator' as a vector of
        [pitch, Rrotation, Dsupport, section_profile] */
-       
-    table[search([designator], table, num_returns_per_match=1,
-                   index_col_num=0)[0]][1];
+      
+    // first lookup designator in table inside a let() statement:
+    let (specs = table[search([designator], table, num_returns_per_match=1,
+                              index_col_num=0)[0]][1])
+        // verify that we found something and return it:
+        assert(!is_undef(specs), str("Designator: '", designator, "' not found")) specs;
 
 module thread(designator, turns, higbee_arc=20, fn=120, table=THREAD_TABLE)
 {