diff --git a/Lib/fontTools/cffLib/specializer.py b/Lib/fontTools/cffLib/specializer.py
index 87da279af..3db5b3882 100644
--- a/Lib/fontTools/cffLib/specializer.py
+++ b/Lib/fontTools/cffLib/specializer.py
@@ -459,19 +459,25 @@ def _convertToBlendCmds(args):
             num_sources = len(arg) - 1
             blendlist = [arg]
             stack_use += 1 + num_sources  # 1 for the num_blends arg
-            while (i < num_args) and isinstance(args[i], list):
+
+            # if we are here, max stack is the CFF2 max stack.
+            # I use the CFF2 max stack limit here rather than
+            # the 'maxstack' chosen by the client, as the default
+            # maxstack may have been used unintentionally. For all
+            # the other operators, this just produces a little less
+            # optimization, but here it puts a hard (and low) limit
+            # on the number of source fonts that can be used.
+            #
+            # Make sure the stack depth does not exceed (maxstack - 1), so
+            # that subroutinizer can insert subroutine calls at any point.
+            while (
+                (i < num_args)
+                and isinstance(args[i], list)
+                and stack_use + num_sources < maxStackLimit
+            ):
                 blendlist.append(args[i])
                 i += 1
                 stack_use += num_sources
-                if stack_use + num_sources > maxStackLimit:
-                    # if we are here, max stack is the CFF2 max stack.
-                    # I use the CFF2 max stack limit here rather than
-                    # the 'maxstack' chosen by the client, as the default
-                    #  maxstack may have been used unintentionally. For all
-                    # the other operators, this just produces a little less
-                    # optimization, but here it puts a hard (and low) limit
-                    # on the number of source fonts that can be used.
-                    break
             # blendList now contains as many single blend tuples as can be
             # combined without exceeding the CFF2 stack limit.
             num_blends = len(blendlist)