diff --git a/CHANGES b/CHANGES
index 32b7c8cbc2..af9f1b88f5 100644
--- a/CHANGES
+++ b/CHANGES
@@ -1,4 +1,9 @@
 
+2.5-842 | 2018-08-15 11:00:20 -0500
+
+  * Fix seg fault on trying to type-cast invalid/nil Broker::Data
+    (Jon Siwek, Corelight)
+
 2.5-841 | 2018-08-14 16:45:09 -0500
 
   * BIT-1798: fix PPTP GRE tunnel decapsulation (Jon Siwek, Corelight)
diff --git a/VERSION b/VERSION
index 7a93a5255d..4744b3cd09 100644
--- a/VERSION
+++ b/VERSION
@@ -1 +1 @@
-2.5-841
+2.5-842
diff --git a/src/Expr.cc b/src/Expr.cc
index f958c63ecf..07034db1a8 100644
--- a/src/Expr.cc
+++ b/src/Expr.cc
@@ -16,6 +16,8 @@
 #include "Trigger.h"
 #include "IPAddr.h"
 
+#include "broker/Data.h"
+
 const char* expr_name(BroExprTag t)
 	{
 	static const char* expr_names[int(NUM_EXPRS)] = {
@@ -5503,11 +5505,16 @@ Val* CastExpr::Eval(Frame* f) const
 		}
 
 	ODesc d;
-	d.Add("cannot cast value of type '");
+	d.Add("invalid cast of value with type '");
 	v->Type()->Describe(&d);
 	d.Add("' to type '");
 	Type()->Describe(&d);
 	d.Add("'");
+
+	if ( same_type(v->Type(), bro_broker::DataVal::ScriptDataType()) &&
+		 ! v->AsRecordVal()->Lookup(0) )
+		d.Add(" (nil $data field)");
+
 	Unref(v);
 	reporter->ExprRuntimeError(this, "%s", d.Description());
 	return 0;  // not reached.
diff --git a/src/Val.cc b/src/Val.cc
index 1024251418..7879d282b2 100644
--- a/src/Val.cc
+++ b/src/Val.cc
@@ -3632,6 +3632,10 @@ Val* cast_value_to_type(Val* v, BroType* t)
 	if ( same_type(v->Type(), bro_broker::DataVal::ScriptDataType()) )
 		{
 		auto dv = v->AsRecordVal()->Lookup(0);
+
+		if ( ! dv )
+			return 0;
+
 		return static_cast<bro_broker::DataVal *>(dv)->castTo(t);
 		}
 
@@ -3654,6 +3658,10 @@ bool can_cast_value_to_type(const Val* v, BroType* t)
 	if ( same_type(v->Type(), bro_broker::DataVal::ScriptDataType()) )
 		{
 		auto dv = v->AsRecordVal()->Lookup(0);
+
+		if ( ! dv )
+			return false;
+
 		return static_cast<const bro_broker::DataVal *>(dv)->canCastTo(t);
 		}
 
diff --git a/testing/btest/Baseline/language.type-cast-error-dynamic/output b/testing/btest/Baseline/language.type-cast-error-dynamic/output
index 10d92ac199..8ebf0cc90e 100644
--- a/testing/btest/Baseline/language.type-cast-error-dynamic/output
+++ b/testing/btest/Baseline/language.type-cast-error-dynamic/output
@@ -1,2 +1,4 @@
-expression error in /home/robin/bro/lang-ext/testing/btest/.tmp/language.type-cast-error-dynamic/type-cast-error-dynamic.bro, line 11: cannot cast value of type 'count' to type 'string' [a as string]
-expression error in /home/robin/bro/lang-ext/testing/btest/.tmp/language.type-cast-error-dynamic/type-cast-error-dynamic.bro, line 11: cannot cast value of type 'record { a:addr; b:port; }' to type 'string' [a as string]
+expression error in /Users/jon/projects/bro/bro/testing/btest/.tmp/language.type-cast-error-dynamic/type-cast-error-dynamic.bro, line 11: invalid cast of value with type 'count' to type 'string' [a as string]
+expression error in /Users/jon/projects/bro/bro/testing/btest/.tmp/language.type-cast-error-dynamic/type-cast-error-dynamic.bro, line 11: invalid cast of value with type 'record { a:addr; b:port; }' to type 'string' [a as string]
+expression error in /Users/jon/projects/bro/bro/testing/btest/.tmp/language.type-cast-error-dynamic/type-cast-error-dynamic.bro, line 11: invalid cast of value with type 'record { data:opaque of Broker::Data; }' to type 'string' (nil $data field) [a as string]
+data is string, F
diff --git a/testing/btest/language/type-cast-error-dynamic.bro b/testing/btest/language/type-cast-error-dynamic.bro
index 45f1d1fb5f..91fa212ce4 100644
--- a/testing/btest/language/type-cast-error-dynamic.bro
+++ b/testing/btest/language/type-cast-error-dynamic.bro
@@ -18,6 +18,8 @@ event bro_init()
 
 	cast_to_string(42);
 	cast_to_string(x);
+	cast_to_string(Broker::Data());
+	print "data is string", Broker::Data() is string;
 	}