diff --git a/src/robitax.c b/src/robitax.c
index 8445e3e..2c505e1 100644
--- a/src/robitax.c
+++ b/src/robitax.c
@@ -622,7 +622,12 @@ SEXP R_is_under_taxon(SEXP Rtaxonomy, SEXP Rtaxid, SEXP Rparent)
               prep[i]=NA_LOGICAL;
             else {
               taxon = eco_findtaxonbytaxid(ptax, ptaxid[i]);
-              prep[i] = eco_isundertaxon(taxon, parent);            
+              if (taxon) {
+                prep[i] = eco_isundertaxon(taxon, parent);
+              }
+              else {
+                prep[i]=NA_LOGICAL;
+              }
             }  
           }          
         }