Index: generic/tclStringObj.c
==================================================================
--- generic/tclStringObj.c
+++ generic/tclStringObj.c
@@ -2943,20 +2943,36 @@
 		    }
 		}
 	    }
 	} while (--oc);
     } else {
+	Tcl_Obj *pendingPtr = NULL;
+
 	/* Result will be concat of string reps. Pre-size it. */
 	ov = objv; oc = objc;
 	do {
 	    int numBytes;
 
 	    objPtr = *ov++;
 
-	    Tcl_GetStringFromObj(objPtr, &numBytes);	/* PANIC? */
-	    if (numBytes) {
+	    if ((length == 0) && (objPtr->bytes == NULL) && !pendingPtr) {
+		/* No string rep; Take the chance we can avoid making it */
+
+		last = objc - oc;
+		first = last;
+		pendingPtr = objPtr;
+	    } else {
+
+		Tcl_GetStringFromObj(objPtr, &numBytes); /* PANIC? */
+		if (numBytes == 0) {
+		    continue;
+		}
 		last = objc - oc;
+		if (pendingPtr) {
+		    Tcl_GetStringFromObj(pendingPtr, &length); /* PANIC? */
+		    pendingPtr = NULL;
+		}
 		if (length == 0) {
 		    first = last;
 		}
 		if ((length += numBytes) < 0) {
 		    goto overflow;
@@ -3073,10 +3089,12 @@
 		return TCL_ERROR;
 	    }
 	    dst = Tcl_GetString(objResultPtr) + start;
 	    if (length > start) {
 		TclFreeIntRep(objResultPtr);
+	    } else {
+		/* Can't happen ? */
 	    }
 	} else {
 	    objResultPtr = Tcl_NewObj();	/* PANIC? */
 	    if (0 == Tcl_AttemptSetObjLength(objResultPtr, length)) {
 		if (interp) {

Index: tests/string.test
==================================================================
--- tests/string.test
+++ tests/string.test
@@ -1992,10 +1992,40 @@
     set vvs [string repeat {$x } $n]
     set r1 [string compare $xx [subst $vv]]
     set r2 [string compare $xx [eval "string cat $vvs"]]
     list $r1 $r2
 } {0 0}
+test string-29.5 {string cat, efficiency} -body {
+    tcl::unsupported::representation [string cat [list x] [list]]
+} -match glob -result {*no string representation}
+test string-29.6 {string cat, efficiency} -body {
+    tcl::unsupported::representation [string cat [list] [list x]]
+} -match glob -result {*no string representation}
+test string-29.7 {string cat, efficiency} -body {
+    tcl::unsupported::representation [string cat [list x] [list] [list]]
+} -match glob -result {*no string representation}
+test string-29.8 {string cat, efficiency} -body {
+    tcl::unsupported::representation [string cat [list] [list x] [list]]
+} -match glob -result {*no string representation}
+test string-29.9 {string cat, efficiency} -body {
+    tcl::unsupported::representation [string cat [list] [list] [list x]]
+} -match glob -result {*no string representation}
+test string-29.10 {string cat, efficiency} -body {
+    tcl::unsupported::representation [string cat [list x] [list x]]
+} -match glob -result {*, string representation "xx"}
+test string-29.11 {string cat, efficiency} -body {
+    tcl::unsupported::representation \
+	[string cat [list x] [encoding convertto utf-8 {}]]
+} -match glob -result {*no string representation}
+test string-29.12 {string cat, efficiency} -body {
+    tcl::unsupported::representation \
+	[string cat [encoding convertto utf-8 {}] [list x]]
+} -match glob -result {*, string representation "x"}
+test string-29.13 {string cat, efficiency} -body {
+    tcl::unsupported::representation [string cat \
+	[encoding convertto utf-8 {}] [encoding convertto utf-8 {}] [list x]]
+} -match glob -result {*, string representation "x"}
 
 
 
 # cleanup
 rename MemStress {}