diff --git a/tren.py b/tren.py
index d6df15d..beea8a1 100755
--- a/tren.py
+++ b/tren.py
@@ -8,7 +8,7 @@
 PROGNAME = "tren.py"
 BASENAME = PROGNAME.split(".py")[0]
 PROGENV  = BASENAME.upper()
-RCSID = "$Id: tren.py,v 1.159 2010/02/26 00:24:54 tundra Exp $"
+RCSID = "$Id: tren.py,v 1.160 2010/02/26 06:40:00 tundra Exp $"
 VERSION = RCSID.split()[2]
 
 # Copyright Information
@@ -489,15 +489,31 @@
 
                     todo = []
 
-                    # Handle single instance requests
+                    # Handle single instance requests doing bounds checking as we go
                     
-                    if (renrequest[INSTANCEEND] == SINGLEINST) and (renrequest[INSTANCESTART] < len(oldstrings)):
-                        todo.append(oldstrings[renrequest[INSTANCESTART]])
+                    start = renrequest[INSTANCESTART]
+                    end   = renrequest[INSTANCEEND]
+
+                    if (end == SINGLEINST):
+                        
+                        # Compute bounds for positive and negative indicies.
+                        # This is necessary because positive indicies are 0-based,
+                        # but negative indicies start at -1.
+
+                        bound = len(oldstrings)
+
+                        if start < 0:
+                            bound += 1
+
+                        # Now go get that entry
+
+                        if abs(start) < bound:
+                            todo.append(oldstrings[start])
 
                     # Handle instance range requests
 
                     else:
-                        todo = oldstrings[renrequest[INSTANCESTART] : renrequest[INSTANCEEND]]
+                        todo = oldstrings[start:end]
 
 
                     # Replace selected substring(s).