diff --git a/jbmc/regression/jbmc-strings/StringConcat/Test.class b/jbmc/regression/jbmc-strings/StringConcat/Test.class
index 7600fbe19d0..b1d54f1ccb5 100644
Binary files a/jbmc/regression/jbmc-strings/StringConcat/Test.class and b/jbmc/regression/jbmc-strings/StringConcat/Test.class differ
diff --git a/jbmc/regression/jbmc-strings/StringConcat/Test.java b/jbmc/regression/jbmc-strings/StringConcat/Test.java
index 74f91b81df0..6ea819f3eff 100644
--- a/jbmc/regression/jbmc-strings/StringConcat/Test.java
+++ b/jbmc/regression/jbmc-strings/StringConcat/Test.java
@@ -312,4 +312,15 @@ public String bufferNonDetLoop5(int cols, int columnWidth, String c, String data
         assert(false);
         return sb.toString();
     }
+
+    static boolean fromNonDetArray(String[] argv)
+    {
+        String s = argv[0];
+        String u = s.concat("llo");
+        if(u.equals("Hello")) {
+            return true;
+        }
+        return false;
+    }
+
 }
diff --git a/jbmc/regression/jbmc-strings/StringConcat/test_string_nondet_array.desc b/jbmc/regression/jbmc-strings/StringConcat/test_string_nondet_array.desc
new file mode 100644
index 00000000000..28307c48203
--- /dev/null
+++ b/jbmc/regression/jbmc-strings/StringConcat/test_string_nondet_array.desc
@@ -0,0 +1,8 @@
+CORE
+Test.class
+--function Test.fromNonDetArray --depth 10000 --unwind 5 --throw-runtime-exceptions  --cp `../../../../scripts/format_classpath.sh . ../../../lib/java-models-library/target/core-models.jar` --trace
+^EXIT=10$
+^SIGNAL=0$
+^VERIFICATION FAILED
+line 318 no uncaught exception: FAILURE
+java.lang.ArrayIndexOutOfBoundsException|java.lang.NullPointerException
diff --git a/src/solvers/strings/string_refinement.cpp b/src/solvers/strings/string_refinement.cpp
index ac045d66c10..548ebfbeea1 100644
--- a/src/solvers/strings/string_refinement.cpp
+++ b/src/solvers/strings/string_refinement.cpp
@@ -790,6 +790,13 @@ decision_proceduret::resultt string_refinementt::dec_solve()
   for(const auto &instance : initial_instances)
     add_lemma(instance);
 
+  // All generated strings should have non-negative length
+  for(const auto &string : generator.array_pool.created_strings())
+  {
+    add_lemma(binary_relation_exprt{
+      string.length(), ID_ge, from_integer(0, string.length().type())});
+  }
+
   while((loop_bound_--) > 0)
   {
     dependencies.clean_cache();
@@ -2060,6 +2067,12 @@ exprt string_refinementt::get(const exprt &expr) const
     }
     const auto array = supert::get(current.get());
     const auto index = get(index_expr->index());
+
+    // If the underlying solver does not know about the existence of an array,
+    // it can return nil, which cannot be used in the expression returned here.
+    if(array.is_nil())
+      return index_exprt(current, index);
+
     const exprt unknown =
       from_integer(CHARACTER_FOR_UNKNOWN, index_expr->type());
     if(
@@ -2070,12 +2083,9 @@ exprt string_refinementt::get(const exprt &expr) const
       return sparse_array->to_if_expression(index);
     }
 
-    INVARIANT(
-      array.is_nil() || array.id() == ID_symbol ||
-        array.id() == ID_nondet_symbol,
-      std::string("apart from symbols, array valuations can be interpreted as "
-                  "sparse arrays, id: ") +
-        id2string(array.id()));
+    INVARIANT(array.id() == ID_symbol || array.id() == ID_nondet_symbol,
+              "Apart from symbols, array valuations can be interpreted as "
+              "sparse arrays. Array model : " + array.pretty());
     return index_exprt(array, index);
   }