diff --git a/lslopt/lslfoldconst.py b/lslopt/lslfoldconst.py
index cfb227c..69fa504 100644
--- a/lslopt/lslfoldconst.py
+++ b/lslopt/lslfoldconst.py
@@ -905,7 +905,7 @@ class foldconst(object):
                         rnt = rval.nt
 
                 if optype == 'list' and not (ltype == rtype == 'list'):
-                    if lnt == 'CONST' and not lval.value:
+                    if lnt == 'CONST' and ltype == 'list' and not lval.value:
                         # [] + nonlist  ->  (list)nonlist
                         parent[index] = self.Cast(rval, optype)
                         # node is SEF if rval is
@@ -928,18 +928,26 @@ class foldconst(object):
                 if optype in ('string', 'float', 'list'):
                     # All these types evaluate to boolean False when they are
                     # the neutral addition element.
-                    if lnt == 'CONST' and not lval.value:
-                        # 0. + expr  ->  expr
-                        # "" + expr  ->  expr
-                        # [] + expr  ->  expr
+                    if lnt == 'CONST' and not lval.value and (ltype == rtype
+                       or ltype == 'integer' and rtype == 'float'
+                       or ltype == 'float' and rtype == 'integer'):
+                        # 0  + fval  ->  fval
+                        # 0. + fval  ->  fval
+                        # 0. + ival  ->  fval
+                        # "" + sval  ->  sval
+                        # [] + lval  ->  lval
                         parent[index] = self.Cast(rval, optype)
                         # node is SEF if rval is
                         parent[index].SEF = rval.SEF
                         return
-                    if rnt == 'CONST' and not rval.value:
-                        # expr + 0.  ->  expr
-                        # expr + ""  ->  expr
-                        # expr + []  ->  expr
+                    if rnt == 'CONST' and not rval.value and (rtype == ltype
+                       or rtype == 'integer' and ltype == 'float'
+                       or rtype == 'float' and ltype == 'integer'):
+                        # fval + 0   ->  fval
+                        # fval + 0.  ->  fval
+                        # ival + 0.  ->  fval
+                        # sval + ""  ->  sval
+                        # lval + []  ->  lval
                         parent[index] = self.Cast(lval, optype)
                         # node is SEF if lval is
                         parent[index].SEF = lval.SEF
diff --git a/unit_tests/regression.suite/issue-13.lsl b/unit_tests/regression.suite/issue-13.lsl
new file mode 100644
index 0000000..ae11009
--- /dev/null
+++ b/unit_tests/regression.suite/issue-13.lsl
@@ -0,0 +1,10 @@
+default
+{
+    state_entry()
+    {
+        llParticleSystem("" + llGetPhysicsMaterial());
+        llParticleSystem((list)"" + llGetPhysicsMaterial());
+        llParticleSystem(0 + llGetPhysicsMaterial());
+        llParticleSystem((list)0 + llGetPhysicsMaterial());
+    }
+}
diff --git a/unit_tests/regression.suite/issue-13.out b/unit_tests/regression.suite/issue-13.out
new file mode 100644
index 0000000..72dc38c
--- /dev/null
+++ b/unit_tests/regression.suite/issue-13.out
@@ -0,0 +1,10 @@
+default
+{
+    state_entry()
+    {
+        llParticleSystem("" + llGetPhysicsMaterial());
+        llParticleSystem("" + llGetPhysicsMaterial());
+        llParticleSystem(0 + llGetPhysicsMaterial());
+        llParticleSystem(0 + llGetPhysicsMaterial());
+    }
+}
diff --git a/unit_tests/regression.suite/list-plus-elem.lsl b/unit_tests/regression.suite/list-plus-elem.lsl
index 7ddffbf..505fa44 100644
--- a/unit_tests/regression.suite/list-plus-elem.lsl
+++ b/unit_tests/regression.suite/list-plus-elem.lsl
@@ -23,5 +23,21 @@ default{timer(){
   a = (list)(list)(list)(list)(list)[b] + a;
   a = (list)(list)(list)(list)(list)1 + a;
   a = (list)(list)(list)(list)(list)b + a;
+  a += (llGetPhysicsMaterial() + 0);
+  a += (llGetPhysicsMaterial() + 0.);
+  a += (llGetPhysicsMaterial() + "");
+  a += (llGetPhysicsMaterial() + []);
+  a += (0 + llGetPhysicsMaterial());
+  a += (0. + llGetPhysicsMaterial());
+  a += ("" + llGetPhysicsMaterial());
+  a += ([] + llGetPhysicsMaterial());
+  a += ([] + llGetPhysicsMaterial() + []);
+  a += (0 + []);
+  a += (0. + []);
+  a += ("" + []);
+  a += ([] + 0);
+  a += ([] + 0.);
+  a += ([] + "");
+  a += ([] + []);
   llParticleSystem(a);
 }}
diff --git a/unit_tests/regression.suite/list-plus-elem.out b/unit_tests/regression.suite/list-plus-elem.out
index f0e4e6a..2233ac5 100644
--- a/unit_tests/regression.suite/list-plus-elem.out
+++ b/unit_tests/regression.suite/list-plus-elem.out
@@ -24,6 +24,21 @@ default
         a = b + a;
         a = 1 + a;
         a = b + a;
+        a = a + (llGetPhysicsMaterial() + 0);
+        a = a + (llGetPhysicsMaterial() + ((float)0));
+        a = a + (llGetPhysicsMaterial() + "");
+        a = a + llGetPhysicsMaterial();
+        a = a + (0 + llGetPhysicsMaterial());
+        a = a + (((float)0) + llGetPhysicsMaterial());
+        a = a + ("" + llGetPhysicsMaterial());
+        a = a + llGetPhysicsMaterial();
+        a = a + llGetPhysicsMaterial();
+        a = a + 0;
+        a = a + ((float)0);
+        a = a + "";
+        a = a + 0;
+        a = a + ((float)0);
+        a = a + "";
         llParticleSystem(a);
     }
 }