diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc
index eac13c3fd9..fc51590b20 100644
--- a/src/libexpr/eval.cc
+++ b/src/libexpr/eval.cc
@@ -26,7 +26,7 @@ void EvalState::addPrimOp(const string & name,
 /* Substitute an argument set into the body of a function. */
 static Expr substArgs(Expr body, ATermList formals, Expr arg)
 {
-    ATermMap subs;
+    ATermMap subs(ATgetLength(formals) * 2);
     Expr undefined = makeUndefined();
 
     /* Get the formal arguments. */
diff --git a/src/libexpr/nixexpr.cc b/src/libexpr/nixexpr.cc
index 6fb72687d1..aee57acb11 100644
--- a/src/libexpr/nixexpr.cc
+++ b/src/libexpr/nixexpr.cc
@@ -50,9 +50,10 @@ void ATermMap::copy(const ATermMap & map)
 {
     ATermList keys = map.keys();
 
-    /* !!! adjust allocation for load pct */
+    /* !!! We adjust for the maximum load pct by allocating twice as
+       much.  Probably a bit too much. */
     maxLoadPct = map.maxLoadPct;
-    table = ATtableCreate(ATgetLength(keys), maxLoadPct);
+    table = ATtableCreate(ATgetLength(keys) * 2, maxLoadPct);
     if (!table) throw Error("cannot create ATerm table");
 
     add(map, keys);