diff --git a/src/math/polynomial/algebraic_numbers.cpp b/src/math/polynomial/algebraic_numbers.cpp
index 5244de82c..57ce00e2c 100644
--- a/src/math/polynomial/algebraic_numbers.cpp
+++ b/src/math/polynomial/algebraic_numbers.cpp
@@ -2013,6 +2013,11 @@ namespace algebraic_numbers {
             }
             else {
                 algebraic_cell * c = a.to_algebraic();
+                if (c->m_i == 0) {
+                    // undefined
+                    c->m_i = upm().get_root_id(c->m_p_sz, c->m_p, lower(c)) + 1;
+                }
+                SASSERT(c->m_i > 0);
                 return c->m_i;
             }
         }