--- a/CHANGES.txt	Sun Jan 23 17:34:11 2022 +0100
+++ b/CHANGES.txt	Fri Mar 04 17:35:27 2022 +0100
@@ -12,6 +12,16 @@
 Pre-1.0 Series
 --------------
 
+none (n/a)
+~~~~~~~~~~
+
+- **[bugfix]**
+  The ``{{::DEL::}}`` is not subject to interpolation any more.
+  This fixes the handling of these deletion markers when merging
+  configurations: sometimes they were tried to be interpolated -- and
+  this failed.
+
+
 0.20.4 (2022-01-17)
 ~~~~~~~~~~~~~~~~~~~
 

--- a/configmix/_speedups.c	Sun Jan 23 17:34:11 2022 +0100
+++ b/configmix/_speedups.c	Fri Mar 04 17:35:27 2022 +0100
@@ -34,6 +34,7 @@
     PyObject *STARTTOK;
     PyObject *ENDTOK;
     PyObject *REF_NAMESPACE;
+    PyObject *DEL_VALUE;
 };
 
 
@@ -713,6 +714,7 @@
     PyObject *s;
     PyObject *cache;
 
+    int cmp;
     Py_ssize_t s_len;
     Py_ssize_t idx;
     Py_ssize_t i, j;
@@ -751,6 +753,14 @@
         return NULL;
     }
 
+    cmp = PyUnicode_Compare(s, sstate->DEL_VALUE);
+    if ((cmp < 0) && PyErr_Occurred()) {
+        return NULL;
+    }
+    if (cmp == 0) {
+        return Py_NewRef(s);
+    }
+
     idx = PyUnicode_Find(s, sstate->STARTTOK, 0, s_len, 1);
     if (idx < 0) {
         PyErr_Clear();
@@ -957,6 +967,7 @@
     PyObject *s;
     PyObject *cache = NULL;
 
+    int cmp;
     Py_ssize_t s_len;
     Py_ssize_t start, rest, end;
     PyObject *tmp;
@@ -991,6 +1002,14 @@
         return NULL;
     }
 
+    cmp = PyUnicode_Compare(s, sstate->DEL_VALUE);
+    if ((cmp < 0) && PyErr_Occurred()) {
+        return NULL;
+    }
+    if (cmp == 0) {
+        return Py_NewRef(s);
+    }
+
     start = PyUnicode_Find(s, sstate->STARTTOK, 0, s_len, 1);
     if (start == -2) {
         return NULL;
@@ -1653,6 +1672,12 @@
     }
     PyUnicode_InternInPlace(&(sstate->REF_NAMESPACE));
 
+    sstate->DEL_VALUE = PyUnicode_FromStringAndSize("{{::DEL::}}", 3);
+    if (sstate->DEL_VALUE == NULL) {
+        return -1;
+    }
+    PyUnicode_InternInPlace(&(sstate->DEL_VALUE));
+
     return 0;
 }
 
@@ -1677,6 +1702,7 @@
         Py_VISIT(sstate->STARTTOK);
         Py_VISIT(sstate->ENDTOK);
         Py_VISIT(sstate->REF_NAMESPACE);
+        Py_VISIT(sstate->DEL_VALUE);
     }
     return 0;
 }
@@ -1702,6 +1728,7 @@
         Py_CLEAR(sstate->STARTTOK);
         Py_CLEAR(sstate->ENDTOK);
         Py_CLEAR(sstate->REF_NAMESPACE);
+        Py_CLEAR(sstate->DEL_VALUE);
     }
     return 0;
 }

--- a/configmix/config.py	Sun Jan 23 17:34:11 2022 +0100
+++ b/configmix/config.py	Fri Mar 04 17:35:27 2022 +0100
@@ -29,7 +29,7 @@
 
 from .variables import lookup_varns, lookup_filter
 from .compat import u, uchr, n, str_and_u, PY2
-from .constants import REF_NAMESPACE, NONE_FILTER, EMPTY_FILTER
+from .constants import REF_NAMESPACE, NONE_FILTER, EMPTY_FILTER, DEL_VALUE
 try:
     from ._speedups import (fast_unquote, fast_quote, fast_pathstr2path,
                             _fast_split_ns, _fast_split_filters,
@@ -931,6 +931,8 @@
         len_s = len(s)
         if len_s < 4:
             return s
+        if s == DEL_VALUE:
+            return s
         start = s.find(_STARTTOK, 0)
         if start < 0:
             return s

--- a/configmix/constants.py	Sun Jan 23 17:34:11 2022 +0100
+++ b/configmix/constants.py	Fri Mar 04 17:35:27 2022 +0100
@@ -27,7 +27,10 @@
 
 DEL_VALUE = _u("{{::DEL::}}")
 """Value for configuration items to signal that the corresponding
-key-value is to be deleted when configurations are merged
+key-value is to be deleted when configurations are merged.
+
+Despite having "interpolation" syntax this value will **never** be
+substituted.
 
 """