diff --git a/src/TocBackend.cpp b/src/TocBackend.cpp
index 399278456d..f490c3534a 100644
--- a/src/TocBackend.cpp
+++ b/src/TocBackend.cpp
@@ -27,7 +27,7 @@
 #include "ParIterator.h"
 #include "TextClass.h"
 
-#include "insets/InsetArgument.h"
+#include "insets/InsetText.h"
 
 #include "support/convert.h"
 #include "support/debug.h"
@@ -193,6 +193,18 @@ void TocBuilder::captionItem(DocIterator const & dit, docstring const & s,
 	}
 }
 
+void TocBuilder::argumentItem(docstring const & arg_str)
+{
+	if (stack_.empty() || arg_str.empty())
+		return;
+	TocItem & item = (*toc_)[stack_.top().pos];
+	docstring const & str = item.str();
+	string const & delim =
+		str.empty() ? "" : stack_.top().is_captioned ? ", " : ": ";
+	item.str(str + from_ascii(delim) + arg_str);
+	stack_.top().is_captioned = true;
+}
+
 void TocBuilder::pop()
 {
 	if (!stack_.empty())
diff --git a/src/TocBackend.h b/src/TocBackend.h
index eaa1ce99e2..b851d68546 100644
--- a/src/TocBackend.h
+++ b/src/TocBackend.h
@@ -120,13 +120,15 @@ class TocBuilder
 {
 public:
 	TocBuilder(std::shared_ptr<Toc> const toc);
-	/// When entering a float
+	/// When entering a float or flex (AddToToc)
 	void pushItem(DocIterator const & dit, docstring const & s,
 	              bool output_active, bool is_captioned = false);
-	/// When encountering a caption
+	/// When encountering a float caption
 	void captionItem(DocIterator const & dit, docstring const & s,
 	                 bool output_active);
-	/// When exiting a float
+	/// When encountering an argument (isTocCaption)
+	void argumentItem(docstring const & arg_str);
+	/// When exiting a float or flex
 	void pop();
 private:
 	TocBuilder(){}
diff --git a/src/insets/InsetArgument.cpp b/src/insets/InsetArgument.cpp
index 6ba49c7aef..605286ca20 100644
--- a/src/insets/InsetArgument.cpp
+++ b/src/insets/InsetArgument.cpp
@@ -26,6 +26,7 @@
 #include "ParIterator.h"
 #include "TexRow.h"
 #include "texstream.h"
+#include "TocBackend.h"
 
 #include "support/convert.h"
 #include "support/debug.h"
@@ -106,6 +107,7 @@ void InsetArgument::updateBuffer(ParIterator const & it, UpdateType utype)
 		}
 	}
 	Layout::LaTeXArgMap::const_iterator const lait = args.find(name_);
+	caption_of_toc_ = string();
 	if (lait != args.end()) {
 		docstring label = translateIfPossible((*lait).second.labelstring);
 		docstring striplabel;
@@ -117,6 +119,12 @@ void InsetArgument::updateBuffer(ParIterator const & it, UpdateType utype)
 		decoration_ = (*lait).second.decoration;
 		pass_thru_chars_ = (*lait).second.pass_thru_chars;
 		pass_thru_local_ = false;
+		if (lait->second.is_toc_caption)
+			// empty if AddToToc is not set
+			caption_of_toc_ = insetlayout
+				? it.inset().getLayout().tocType()
+				: it.paragraph().layout().tocType();
+
 		switch ((*lait).second.passthru) {
 			case PT_INHERITED:
 				pass_thru_ = pass_thru_context_;
@@ -309,4 +317,18 @@ void InsetArgument::latexArgument(otexstream & os,
 }
 
 
+
+void InsetArgument::addToToc(DocIterator const & dit, bool output_active,
+                             UpdateType utype) const
+{
+	if (!caption_of_toc_.empty()) {
+		docstring str;
+		text().forOutliner(str, TOC_ENTRY_LENGTH);
+		buffer().tocBackend().builder(caption_of_toc_).argumentItem(str);
+	}
+	// Proceed with the rest of the inset.
+	InsetText::addToToc(dit, output_active, utype);
+}
+
+
 } // namespace lyx
diff --git a/src/insets/InsetArgument.h b/src/insets/InsetArgument.h
index fb7e290aeb..0ccdc41bc0 100644
--- a/src/insets/InsetArgument.h
+++ b/src/insets/InsetArgument.h
@@ -81,6 +81,9 @@ public:
 	///
 	void setButtonLabel();
 	//@}
+	///
+	void addToToc(DocIterator const & dit, bool output_active,
+	              UpdateType utype) const; //override
 
 private:
 	///
@@ -105,6 +108,8 @@ private:
 	bool pass_thru_;
 	///
 	docstring pass_thru_chars_;
+	/// The type of Toc this is the caption of, empty otherwise.
+	std::string caption_of_toc_;
 
 protected:
 	/// \name Protected functions inherited from Inset class
diff --git a/src/insets/InsetFlex.cpp b/src/insets/InsetFlex.cpp
index 18f341789b..05cbb298b2 100644
--- a/src/insets/InsetFlex.cpp
+++ b/src/insets/InsetFlex.cpp
@@ -175,16 +175,14 @@ void InsetFlex::addToToc(DocIterator const & cpit, bool output_active,
 		// Cursor inside the inset
 		DocIterator pit = cpit;
 		pit.push_back(CursorSlice(const_cast<InsetFlex &>(*this)));
-		docstring str;
-		str = getLabel();
-		if (layout.isTocCaption()) {
-			if (!str.empty())
-				str += ": ";
-			text().forOutliner(str, TOC_ENTRY_LENGTH);
-		}
-		b.pushItem(pit, str, output_active);
+		b.pushItem(pit, getLabel(), output_active);
 		// Proceed with the rest of the inset.
 		InsetCollapsable::addToToc(cpit, output_active, utype);
+		if (layout.isTocCaption()) {
+			docstring str;
+			text().forOutliner(str, TOC_ENTRY_LENGTH);
+			b.argumentItem(str);
+		}
 		b.pop();
 	} else
 		InsetCollapsable::addToToc(cpit, output_active, utype);