diff --git a/src/rose/rose_graph.h b/src/rose/rose_graph.h
index 499d796ac..b5bf1985d 100644
--- a/src/rose/rose_graph.h
+++ b/src/rose/rose_graph.h
@@ -112,7 +112,7 @@ struct LeftEngInfo {
     }
     size_t hash() const;
     void reset(void);
-    operator bool() const;
+    explicit operator bool() const;
     bool tracksSom() const { return !!haig; }
 };
 
@@ -133,7 +133,7 @@ struct RoseSuffixInfo {
     bool operator<(const RoseSuffixInfo &b) const;
     size_t hash() const;
     void reset(void);
-    operator bool() const { return graph || castle || haig || rdfa || tamarama; }
+    explicit operator bool() const { return graph || castle || haig || rdfa || tamarama; }
 };
 
 /** \brief Properties attached to each Rose graph vertex. */
diff --git a/src/util/ue2_graph.h b/src/util/ue2_graph.h
index b8e2e935d..b6fe1e368 100644
--- a/src/util/ue2_graph.h
+++ b/src/util/ue2_graph.h
@@ -176,7 +176,7 @@ class vertex_descriptor : totally_ordered<vertex_descriptor<Graph>> {
     vertex_descriptor() : p(nullptr), serial(0) {}
     explicit vertex_descriptor(vertex_node *pp) : p(pp), serial(pp->serial) {}
 
-    operator bool() const { return p; }
+    explicit operator bool() const { return p; }
     bool operator<(const vertex_descriptor b) const {
         if (p && b.p) {
             /* no vertices in the same graph can have the same serial */
@@ -215,7 +215,7 @@ class edge_descriptor : totally_ordered<edge_descriptor<Graph>> {
         assert(tup.second == (bool)tup.first);
     }
 
-    operator bool() const { return p; }
+    explicit operator bool() const { return p; }
     bool operator<(const edge_descriptor b) const {
         if (p && b.p) {
             /* no edges in the same graph can have the same serial */