Index: generic/nsf.c
===================================================================
diff -u -r54277a59a45fb6f055637b9a0e14a2fefd31f7eb -r9bd2c31a189868ce6564005d7b5a7cf9e0dc81a5
--- generic/nsf.c	(.../nsf.c)	(revision 54277a59a45fb6f055637b9a0e14a2fefd31f7eb)
+++ generic/nsf.c	(.../nsf.c)	(revision 9bd2c31a189868ce6564005d7b5a7cf9e0dc81a5)
@@ -26063,7 +26063,7 @@
   }
 
   result = Tcl_ListObjGetElements(interp, arglistObj, &objc, &objv);
-  if (likely(result == TCL_OK)) {
+  if (likely(result == TCL_OK) && parsedParam.paramDefs != NULL) {
     ParseContext  pc;
     NsfParamDefs *paramDefs = parsedParam.paramDefs;
     unsigned int  processFlags = 0u;
Index: tests/parameters.test
===================================================================
diff -u -r54277a59a45fb6f055637b9a0e14a2fefd31f7eb -r9bd2c31a189868ce6564005d7b5a7cf9e0dc81a5
--- tests/parameters.test	(.../parameters.test)	(revision 54277a59a45fb6f055637b9a0e14a2fefd31f7eb)
+++ tests/parameters.test	(.../parameters.test)	(revision 9bd2c31a189868ce6564005d7b5a7cf9e0dc81a5)
@@ -3371,6 +3371,13 @@
     ? {apply {{} {nsf::parseargs a 1; info exists a}}} "1"
     ? {apply {{} {nsf::parseargs {a b} {1 2}; expr {[info exists a]+[info exists b]}}}} "2"
     ? {apply {{} {nsf::parseargs {a b args} {1 2 3 4}; expr {[info exists a]+[info exists b]+[info exists args]}}}} "3"
+
+    ? {apply {{} {nsf::parseargs a 1; expr {"a" in [info vars]};}}} 1
+    ? {apply {{} {nsf::parseargs {} {}; llength [info vars];}}} 0
+
+    # TODO: Are the below cases intended?
+    ? {apply {{} {nsf::parseargs {a} {}; llength [info vars];}}} 0
+    ? {apply {{} {nsf::parseargs {} {1}; llength [info vars];}}} 0
   }
 
 #