Index: generic/tkCanvArc.c
==================================================================
--- generic/tkCanvArc.c
+++ generic/tkCanvArc.c
@@ -11,10 +11,12 @@
  */
 
 #include "tkInt.h"
 #include "tkCanvas.h"
 
+#include "float.h"
+
 /*
  * The structure below defines the record for each arc item.
  */
 
 typedef enum {
@@ -181,11 +183,11 @@
 
 static void		ComputeArcBbox(Tk_Canvas canvas, ArcItem *arcPtr);
 static int		ConfigureArc(Tcl_Interp *interp,
 			    Tk_Canvas canvas, Tk_Item *itemPtr, int objc,
 			    Tcl_Obj *const objv[], int flags);
-static void		ComputeArcFromHeight(ArcItem *arcPtr);
+static void		ComputeArcParametersFromHeight(ArcItem *arcPtr);
 static int		CreateArc(Tcl_Interp *interp,
 			    Tk_Canvas canvas, struct Tk_Item *itemPtr,
 			    int objc, Tcl_Obj *const objv[]);
 static void		DeleteArc(Tk_Canvas canvas,
 			    Tk_Item *itemPtr, Display *display);
@@ -467,17 +469,16 @@
     } else {
 	itemPtr->redraw_flags &= ~TK_ITEM_STATE_DEPENDANT;
     }
 
     /*
-     * If either the height is provided then the start and extent will be
-     * overridden.
+     * Override the start and extent if the height is given.
      */
-    if (arcPtr->height != 0) {
-	ComputeArcFromHeight(arcPtr);
-	ComputeArcBbox(canvas, arcPtr);
-    }
+
+    ComputeArcParametersFromHeight(arcPtr);
+    
+    ComputeArcBbox(canvas, arcPtr);
 
     i = (int) (arcPtr->start/360.0);
     arcPtr->start -= i*360.0;
     if (arcPtr->start < 0) {
 	arcPtr->start += 360.0;
@@ -587,11 +588,11 @@
 }
 
 /*
  *--------------------------------------------------------------
  *
- * ComputeArcFromHeight --
+ * ComputeArcParametersFromHeight --
  *
  *	This function calculates the arc parameters given start-point,
  *	end-point and height (!= 0).
  *
  * Results:
@@ -602,21 +603,34 @@
  *
  *--------------------------------------------------------------
  */
 
 static void
-ComputeArcFromHeight(
+ComputeArcParametersFromHeight(
     ArcItem* arcPtr)
 {
     double chordLen, chordDir[2], chordCen[2], arcCen[2], d, radToDeg, radius;
 
     /*
-     * The chord.
+     * Do nothing if no height has been specified.
+     */
+
+    if (arcPtr->height == 0)
+        return;
+
+    /*
+     * Calculate the chord length, return early if it is too small.
      */
 
     chordLen = hypot(arcPtr->endPoint[1] - arcPtr->startPoint[1],
 	    arcPtr->startPoint[0] - arcPtr->endPoint[0]);
+
+    if (chordLen < DBL_EPSILON) {
+        arcPtr->start = arcPtr->extent = arcPtr->height = 0;
+        return;
+    }
+
     chordDir[0] = (arcPtr->endPoint[0] - arcPtr->startPoint[0]) / chordLen;
     chordDir[1] = (arcPtr->endPoint[1] - arcPtr->startPoint[1]) / chordLen;
     chordCen[0] = (arcPtr->startPoint[0] + arcPtr->endPoint[0]) / 2;
     chordCen[1] = (arcPtr->startPoint[1] + arcPtr->endPoint[1]) / 2;
 
@@ -789,11 +803,11 @@
     }
 
     ComputeArcOutline(canvas,arcPtr);
 
     /*
-     * To compute the bounding box, start with the the bbox formed by the two
+     * To compute the bounding box, start with the bbox formed by the two
      * endpoints of the arc. Then add in the center of the arc's oval (if
      * relevant) and the 3-o'clock, 6-o'clock, 9-o'clock, and 12-o'clock
      * positions, if they are relevant.
      */
 

Index: tests/canvas.test
==================================================================
--- tests/canvas.test
+++ tests/canvas.test
@@ -1037,10 +1037,28 @@
         {#c0c0c0 #c0c0c0 #c0c0c0 #c0c0c0 #c0c0c0 #c0c0c0 #c0c0c0 #c0c0c0 #c0c0c0 #c0c0c0 #c0c0c0 #c0c0c0 #c0c0c0 #c0c0c0 #c0c0c0 #c0c0c0 #c0c0c0 #c0c0c0 #c0c0c0 #c0c0c0}}
 } -cleanup {
     destroy .c
     image delete testimage
 } -result 1
+
+test canvas-21.1 {canvas very small arc} -setup {
+    catch {destroy .c}
+    canvas .c
+} -body {
+    # no Inf or NaN must be generated even for very small arcs
+    .c create arc 0 100 0 100 -height 100 -style arc -outline "" -tags arc1
+    set arcBox [.c bbox arc1]
+    .c create arc 0 100 0 100 -height 100 -style arc -outline blue -tags arc2
+    set outlinedArcBox [.c bbox arc2]
+    set coords [.c coords arc1]
+    set start [.c itemcget arc1 -start]
+    set extent [.c itemcget arc1 -extent]
+    set width [.c itemcget arc1 -width]
+    set height [.c itemcget arc1 -height]
+    list $arcBox $outlinedArcBox $coords $start $extent $width $height
+} -result {{-1 99 1 101} {-2 98 2 102} {0.0 100.0 0.0 100.0} 0.0 0.0 1.0 0.0}
+
 
 # cleanup
 imageCleanup
 cleanupTests
 return