diff --git a/site_ansto/instrument/util/motor_utility.tcl b/site_ansto/instrument/util/motor_utility.tcl
index d40f6363..815a2b0a 100644
--- a/site_ansto/instrument/util/motor_utility.tcl
+++ b/site_ansto/instrument/util/motor_utility.tcl
@@ -54,7 +54,15 @@ publish reldrive user
 proc prun {args} {
   if [ catch {
     foreach {mot pname} $args {
-      lappend drlist $mot [SplitReply [$mot posit2unit $pname]]  
+      set posnames [SplitReply [$mot position_names]]
+      if {([llength $posnames] == 0) && [string is double $pname]} {
+        lappend drlist $mot $pname
+      } else {
+        if {[lsearch $posnames $pname] == -1} {
+          error "$pname must be one of $posnames for $mot"
+        }
+        lappend drlist $mot [SplitReply [$mot posit2unit $pname]]
+      }
     }
     clientput run {*}$drlist
     anstocapture run {*}$drlist
@@ -75,7 +83,15 @@ publish prun user
 proc pdrive {args} {
   if [ catch {
     foreach {mot pname} $args {
-      lappend drlist $mot [SplitReply [$mot posit2unit $pname]]  
+      set posnames [SplitReply [$mot position_names]]
+      if {([llength $posnames] == 0) && [string is double $pname]} {
+        lappend drlist $mot $pname
+      } else {
+        if {[lsearch $posnames $pname] == -1} {
+          error "$pname must be one of $posnames for $mot"
+        }
+        lappend drlist $mot [SplitReply [$mot posit2unit $pname]]
+      }
     }
     clientput drive {*}$drlist
     anstocapture drive {*}$drlist