diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc
index 834b15cbd..ce38e2ab9 100644
--- a/src/libexpr/eval.cc
+++ b/src/libexpr/eval.cc
@@ -87,7 +87,7 @@ static Expr substArgs(EvalState & state,
            arguments.  Find out which. */
         for (ATermIterator i(formals); i; ++i) {
             Expr name; ATerm d1, d2;
-            matchFormal(*i, name, d1, d2);
+            if (!matchFormal(*i, name, d1, d2)) abort();
             subs.remove(name);
         }
         throw TypeError(format("the function does not expect an argument named `%1%'")
diff --git a/src/libexpr/nixexpr.cc b/src/libexpr/nixexpr.cc
index 06ca8c928..fac2c0e5c 100644
--- a/src/libexpr/nixexpr.cc
+++ b/src/libexpr/nixexpr.cc
@@ -130,7 +130,7 @@ Expr substitute(const Substitution & subs, Expr e)
     /* In case of a function, filter out all variables bound by this
        function. */
     ATermList formals;
-    ATerm body, def;
+    ATerm body;
     if (matchFunction(e, formals, body, pos)) {
         ATermMap map(ATgetLength(formals));
         for (ATermIterator i(formals); i; ++i) {
@@ -227,7 +227,7 @@ static void checkVarDefs2(set<Expr> & done, const ATermMap & defs, Expr e)
         for (ATermIterator i(formals); i; ++i) {
             ATerm valids, deflt;
             set<Expr> done2;
-            matchFormal(*i, name, valids, deflt);
+            if (!matchFormal(*i, name, valids, deflt)) abort();
             checkVarDefs2(done, defs, valids);
             checkVarDefs2(done2, defs2, deflt);
         }
diff --git a/src/libexpr/nixexpr.hh b/src/libexpr/nixexpr.hh
index 3fd22ecfe..28ac35e60 100644
--- a/src/libexpr/nixexpr.hh
+++ b/src/libexpr/nixexpr.hh
@@ -50,7 +50,7 @@ struct Substitution
     {
         Expr x;
         for (const Substitution * s(this); s; s = s->prev)
-            if (x = s->map->get(name)) return x;
+            if ((x = s->map->get(name))) return x;
         return 0;
     }
 };
diff --git a/src/libexpr/parser.cc b/src/libexpr/parser.cc
index 7adbcb618..9b3e9041d 100644
--- a/src/libexpr/parser.cc
+++ b/src/libexpr/parser.cc
@@ -75,7 +75,7 @@ Expr unescapeStr(const char * s)
 {
     string t;
     char c;
-    while (c = *s++) {
+    while ((c = *s++)) {
         if (c == '\\') {
             assert(*s);
             c = *s++;