diff --git a/osh/cmd_eval.py b/osh/cmd_eval.py
index 1bec67528..b9dc2edf8 100644
--- a/osh/cmd_eval.py
+++ b/osh/cmd_eval.py
@@ -208,7 +208,7 @@ def PlusEquals(old_val, val):
                 str_to_append = cast(value.Str, UP_val)
                 val = value.Str(old_val.s + str_to_append.s)
 
-            elif tag in (value_e.BashArray, value_e.SparseArray):
+            elif tag in (value_e.BashArray, value_e.SparseArray, value_e.BashAssoc):
                 e_die("Can't append array to string")
 
             else:
@@ -236,6 +236,9 @@ def PlusEquals(old_val, val):
                     bash_impl.SparseArray_AppendValues(sparse_lhs, strs)
                     val = sparse_lhs
 
+            elif tag == value_e.BashAssoc:
+                e_die("Can't append an associative array to an indexed array")
+
             else:
                 raise AssertionError()  # parsing should prevent this
 
@@ -243,6 +246,9 @@ def PlusEquals(old_val, val):
             if tag == value_e.Str:
                 e_die("Can't append string to associative arrays")
 
+            elif tag in (value_e.BashArray, value_e.SparseArray):
+                e_die("Can't append an assoxiative array to indexed arrays")
+
             elif tag == value_e.BashAssoc:
                 assoc_lhs = cast(value.BashAssoc, UP_old_val)
                 assoc_rhs = cast(value.BashAssoc, UP_val)
diff --git a/spec/append.test.sh b/spec/append.test.sh
index 5e90478d2..cb2ba852e 100644
--- a/spec/append.test.sh
+++ b/spec/append.test.sh
@@ -301,3 +301,29 @@ echo "${e[@]}"
 ## N-I bash STDOUT:
 e x
 ## END
+
+
+#### Type mismatching of lhs+=rhs should not cause a crash
+case $SH in mksh|zsh) exit ;; esac
+s=
+a=()
+declare -A d=([lemon]=yellow)
+
+s+=(1)
+s+=([melon]=green)
+
+a+=lime
+a+=([1]=banana)
+
+d+=orange
+d+=(0)
+
+true
+
+## STDOUT:
+## END
+
+## OK osh status: 1
+
+## N-I mksh/zsh STDOUT:
+## END