diff --git a/src/EventHandler.h b/src/EventHandler.h
index 2acd2569a6..6006f2e48e 100644
--- a/src/EventHandler.h
+++ b/src/EventHandler.h
@@ -102,7 +102,7 @@ public:
 
 	EventHandler* Ptr()	{ return handler; }
 
-	operator bool() const	{ return handler && *handler; }
+	explicit operator bool() const	{ return handler && *handler; }
 	EventHandler* operator->()	{ return handler; }
 	const EventHandler* operator->() const	{ return handler; }
 
diff --git a/src/UID.h b/src/UID.h
index 2cda02811f..3a32b843f0 100644
--- a/src/UID.h
+++ b/src/UID.h
@@ -61,7 +61,7 @@ public:
 	 *         and not yet initialized w/ Set().
 	 * TODO: this would be better as an "explicit" conversion operator (C++11)
 	 */
-	operator bool() const
+	explicit operator bool() const
 		{ return initialized; }
 
 	/**
diff --git a/src/analyzer/Tag.h b/src/analyzer/Tag.h
index 9ba04b2ef8..35dc3b2ac5 100644
--- a/src/analyzer/Tag.h
+++ b/src/analyzer/Tag.h
@@ -46,7 +46,7 @@ public:
 	 *       "safe bool" idiom (not necessary if "explicit" is available),
 	 *       otherwise this may allow nonsense/undesired comparison operations.
 	 */
-	operator bool() const	{ return *this != Tag(); }
+	explicit operator bool() const	{ return *this != Tag(); }
 
 	/**
 	 * Assignment operator.
diff --git a/src/analyzer/protocol/sip/sip-analyzer.pac b/src/analyzer/protocol/sip/sip-analyzer.pac
index 829904aa3a..3174e6a977 100644
--- a/src/analyzer/protocol/sip/sip-analyzer.pac
+++ b/src/analyzer/protocol/sip/sip-analyzer.pac
@@ -8,7 +8,7 @@ refine flow SIP_Flow += {
 
 	%init{
 		content_length = 0;
-		build_headers = (sip_all_headers != 0);
+		build_headers = bool(sip_all_headers);
 	%}
 
 	function get_content_length(): int
diff --git a/src/file_analysis/Manager.cc b/src/file_analysis/Manager.cc
index 41528e2ceb..177f5446dd 100644
--- a/src/file_analysis/Manager.cc
+++ b/src/file_analysis/Manager.cc
@@ -448,7 +448,7 @@ bool Manager::IsDisabled(analyzer::Tag tag)
 	if ( ! disabled )
 		disabled = internal_const_val("Files::disable")->AsTableVal();
 
-	Val* index = new Val(tag, TYPE_COUNT);
+	Val* index = new Val(bool(tag), TYPE_COUNT);
 	Val* yield = disabled->Lookup(index);
 	Unref(index);
 
diff --git a/src/file_analysis/Tag.h b/src/file_analysis/Tag.h
index c28183a07f..afa1a22ec0 100644
--- a/src/file_analysis/Tag.h
+++ b/src/file_analysis/Tag.h
@@ -46,7 +46,7 @@ public:
 	 *       otherwise this may allow nonsense/undesired comparison operations.
 	 *
 	 */
-	operator bool() const	{ return *this != Tag(); }
+	explicit operator bool() const	{ return *this != Tag(); }
 
 	/**
 	 * Assignment operator.
diff --git a/src/plugin/Plugin.h b/src/plugin/Plugin.h
index 2cec7cf453..9c5416230b 100644
--- a/src/plugin/Plugin.h
+++ b/src/plugin/Plugin.h
@@ -78,7 +78,7 @@ struct VersionNumber {
 	/**
 	 *  Returns true if the version is set to a non-negative value.
 	 */
-	operator bool() const { return major >= 0 && minor >= 0; }
+	explicit operator bool() const { return major >= 0 && minor >= 0; }
 };
 
 /**