Index: lib/StaticAnalyzer/Core/SimpleSValBuilder.cpp
===================================================================
--- lib/StaticAnalyzer/Core/SimpleSValBuilder.cpp
+++ lib/StaticAnalyzer/Core/SimpleSValBuilder.cpp
@@ -911,8 +911,9 @@
       elementType = elemReg->getElementType();
     }
     else if (isa<SubRegion>(region)) {
+      assert(op == BO_Add || op == BO_Sub);
+      index = (op == BO_Add) ? rhs : evalMinus(rhs);
       superR = region;
-      index = rhs;
       if (resultTy->isAnyPointerType())
         elementType = resultTy->getPointeeType();
     }
Index: test/Analysis/ptr-arith.c
===================================================================
--- test/Analysis/ptr-arith.c
+++ test/Analysis/ptr-arith.c
@@ -296,3 +296,16 @@
   clang_analyzer_eval(&points[i].x < &points[i].y);// expected-warning{{TRUE}}
 }
 
+void negativeIndex(char *str) {
+  char *ptr = str + 1;
+  *ptr = 'a';
+  ptr = str - 1;
+  clang_analyzer_eval(*ptr); // expected-warning{{UNKNOWN}}
+  ptr = str;
+  ptr -= 1;
+  clang_analyzer_eval(*ptr); // expected-warning{{UNKNOWN}}
+  ptr = str;
+  --ptr;
+  clang_analyzer_eval(*ptr); // expected-warning{{UNKNOWN}}
+}
+