fix #2891

Signed-off-by: Nikolaj Bjorner <nbjorner@microsoft.com>

src/ast/rewriter/arith_rewriter.cpp

@@ -1155,6 +1155,9 @@ br_status arith_rewriter::mk_power_core(expr * arg1, expr * arg2, expr_ref & res
         // (^ t -k) --> (^ (/ 1 t) k)
         result = m_util.mk_power(m_util.mk_div(m_util.mk_numeral(rational(1), false), arg1),
                                  m_util.mk_numeral(-y, false));
+        result = m().mk_ite(m().mk_eq(arg1, m_util.mk_real(0)),
+                            m_util.mk_real(1),
+                            result);
         return BR_REWRITE2;
     }