diff --git a/docs/CHANGELOG.txt b/docs/CHANGELOG.txt
index ab05e516a..739a19250 100644
--- a/docs/CHANGELOG.txt
+++ b/docs/CHANGELOG.txt
@@ -76,6 +76,8 @@ Other Changes:
     before layout has been locked (first row or headers row submitted). (#8250)
 - Fonts:
   - imgui_freetype: add FreeType headers & compiled version in 'About Dear ImGui' details.
+  - Assert when using MergeMode with an explicit size over a target font with an implicit
+    size, as scale factor are likely erroneous. (#9361)
 - Clipper:
   - Improved error reporting when misusing the clipper inside a table (prioritize
     reporting the common clipper error over a table sanity check assert). (#9350)
diff --git a/imgui.h b/imgui.h
index e73b5b0d3..fb30f3624 100644
--- a/imgui.h
+++ b/imgui.h
@@ -3861,6 +3861,7 @@ enum ImFontFlags_
     ImFontFlags_NoLoadError             = 1 << 1,   // Disable throwing an error/assert when calling AddFontXXX() with missing file/data. Calling code is expected to check AddFontXXX() return value.
     ImFontFlags_NoLoadGlyphs            = 1 << 2,   // [Internal] Disable loading new glyphs.
     ImFontFlags_LockBakedSizes          = 1 << 3,   // [Internal] Disable loading new baked sizes, disable garbage collecting current ones. e.g. if you want to lock a font to a single size. Important: if you use this to preload given sizes, consider the possibility of multiple font density used on Retina display.
+    ImFontFlags_ImplicitRefSize         = 1 << 4,   // [Internal] Reference size was not set explicitly.
 };
 
 // Font runtime data and rendering
diff --git a/imgui_draw.cpp b/imgui_draw.cpp
index 657e1c512..21e368a43 100644
--- a/imgui_draw.cpp
+++ b/imgui_draw.cpp
@@ -3039,7 +3039,7 @@ ImFont* ImFontAtlas::AddFont(const ImFontConfig* font_cfg_in)
     }
     else
     {
-        IM_ASSERT(Fonts.Size > 0 && "Cannot use MergeMode for the first font"); // When using MergeMode make sure that a font has already been added before.
+        IM_ASSERT(Fonts.Size > 0 && "Cannot use MergeMode for the first font!"); // When using MergeMode make sure that a font has already been added before.
         font = font_cfg_in->DstFont ? font_cfg_in->DstFont : Fonts.back();
         ImFontAtlasFontDiscardBakes(this, font, 0); // Need to discard bakes if the font was already used, because baked->FontLoaderDatas[] will change size. (#9162)
     }
@@ -3067,6 +3067,11 @@ ImFont* ImFontAtlas::AddFont(const ImFontConfig* font_cfg_in)
         IM_ASSERT(font_cfg->FontLoader->FontBakedLoadGlyph != NULL);
         IM_ASSERT(font_cfg->FontLoader->LoaderInit == NULL && font_cfg->FontLoader->LoaderShutdown == NULL); // FIXME-NEWATLAS: Unsupported yet.
     }
+    //                             | Target w/ Implicit RefSize | Target w/ Explicit RefSize |
+    // Adding w/ Implicit RefSize: | OK (same scale)            | OK (same scale)            |
+    // Adding w/ Explicit RefSize: | KO                         | OK (custom scale)          |
+    if (font_cfg_in->MergeMode && font_cfg_in->SizePixels > 0)
+        IM_ASSERT((font->Flags & ImFontFlags_ImplicitRefSize) == 0 && "Cannot use MergeMode with an explicit reference size when the destination font used an implicit reference size!");
     IM_ASSERT(font_cfg->FontLoaderData == NULL);
 
     if (!ImFontAtlasFontSourceInit(this, font_cfg))
@@ -3134,7 +3139,10 @@ ImFont* ImFontAtlas::AddFontDefaultBitmap(const ImFontConfig* font_cfg_template)
     if (!font_cfg_template)
         font_cfg.PixelSnapH = true; // Prevents sub-integer scaling factors at lower-level layers.
     if (font_cfg.SizePixels <= 0.0f)
+    {
         font_cfg.SizePixels = 13.0f; // This only serves (1) as a reference for GlyphOffset.y setting and (2) as a default for pre-1.92 backend.
+        font_cfg.Flags |= ImFontFlags_ImplicitRefSize;
+    }
     if (font_cfg.Name[0] == '\0')
         ImFormatString(font_cfg.Name, IM_COUNTOF(font_cfg.Name), "ProggyClean.ttf");
     font_cfg.EllipsisChar = (ImWchar)0x0085;
@@ -3159,7 +3167,10 @@ ImFont* ImFontAtlas::AddFontDefaultVector(const ImFontConfig* font_cfg_template)
     if (!font_cfg_template)
         font_cfg.PixelSnapH = true; // Precisely match ProggyClean, but prevents sub-integer scaling factors at lower-level layers.
     if (font_cfg.SizePixels <= 0.0f)
+    {
         font_cfg.SizePixels = 13.0f;
+        font_cfg.Flags |= ImFontFlags_ImplicitRefSize;
+    }
     if (font_cfg.Name[0] == '\0')
         ImFormatString(font_cfg.Name, IM_COUNTOF(font_cfg.Name), "ProggyForever.ttf");
     font_cfg.ExtraSizeScale *= 1.015f; // Match ProggyClean
diff --git a/misc/freetype/imgui_freetype.cpp b/misc/freetype/imgui_freetype.cpp
index 76c0ac9bc..3c817d20f 100644
--- a/misc/freetype/imgui_freetype.cpp
+++ b/misc/freetype/imgui_freetype.cpp
@@ -428,8 +428,9 @@ static bool ImGui_ImplFreeType_FontBakedInit(ImFontAtlas* atlas, ImFontConfig* s
 {
     IM_UNUSED(atlas);
     float size = baked->Size;
+    const float ref_size = baked->OwnerFont->Sources[0]->SizePixels;
     if (src->MergeMode && src->SizePixels != 0.0f)
-        size *= (src->SizePixels / baked->OwnerFont->Sources[0]->SizePixels);
+        size *= (src->SizePixels / ref_size);
     size *= src->ExtraSizeScale;
 
     ImGui_ImplFreeType_FontSrcData* bd_font_data = (ImGui_ImplFreeType_FontSrcData*)src->FontLoaderData;