diff --git a/source/blender/bmesh/intern/bmesh_operator_api.hh b/source/blender/bmesh/intern/bmesh_operator_api.hh
index 983cc3e3cfe..f36c924e404 100644
--- a/source/blender/bmesh/intern/bmesh_operator_api.hh
+++ b/source/blender/bmesh/intern/bmesh_operator_api.hh
@@ -12,6 +12,7 @@
 #include "BLI_utildefines.h"
 
 #include <cstdarg>
+#include <optional>
 
 #include "bmesh_class.hh"
 
@@ -239,6 +240,15 @@ union eBMOpSlotSubType_Union {
   eBMOpSlotSubType_Int intg;
 };
 
+enum eBMOpSlotFlag : uint8_t {
+  /**
+   * This flag is set when the operators value has been set.
+   * Use this so it's possible to have non-zero defaults for properties.
+   */
+  BMO_OP_SLOT_FLAG_IS_SET = (1 << 0),
+};
+ENUM_OPERATORS(eBMOpSlotFlag, BMO_OP_SLOT_FLAG_IS_SET)
+
 struct BMO_FlagSet {
   int value;
   const char *identifier;
@@ -252,8 +262,8 @@ struct BMOpSlot {
   eBMOpSlotType slot_type;
   eBMOpSlotSubType_Union slot_subtype;
 
+  eBMOpSlotFlag flag;
   int len;
-  //  int flag;  /* UNUSED */
   //  int index; /* index within slot array */  /* UNUSED */
   union {
     int i;
@@ -549,10 +559,16 @@ void BMO_op_flag_disable(BMesh *bm, BMOperator *op, int op_flag);
 
 void BMO_slot_float_set(BMOpSlot slot_args[BMO_OP_MAX_SLOTS], const char *slot_name, float f);
 float BMO_slot_float_get(BMOpSlot slot_args[BMO_OP_MAX_SLOTS], const char *slot_name);
+std::optional<float> BMO_slot_float_get_optional(BMOpSlot slot_args[BMO_OP_MAX_SLOTS],
+                                                 const char *slot_name);
 void BMO_slot_int_set(BMOpSlot slot_args[BMO_OP_MAX_SLOTS], const char *slot_name, int i);
 int BMO_slot_int_get(BMOpSlot slot_args[BMO_OP_MAX_SLOTS], const char *slot_name);
+std::optional<int> BMO_slot_int_get_optional(BMOpSlot slot_args[BMO_OP_MAX_SLOTS],
+                                             const char *slot_name);
 void BMO_slot_bool_set(BMOpSlot slot_args[BMO_OP_MAX_SLOTS], const char *slot_name, bool i);
 bool BMO_slot_bool_get(BMOpSlot slot_args[BMO_OP_MAX_SLOTS], const char *slot_name);
+std::optional<bool> BMO_slot_bool_get_optional(BMOpSlot slot_args[BMO_OP_MAX_SLOTS],
+                                               const char *slot_name);
 /**
  * Return a copy of the element buffer.
  */
diff --git a/source/blender/bmesh/intern/bmesh_operators.cc b/source/blender/bmesh/intern/bmesh_operators.cc
index d56a9b80f5a..d93dc25ac26 100644
--- a/source/blender/bmesh/intern/bmesh_operators.cc
+++ b/source/blender/bmesh/intern/bmesh_operators.cc
@@ -295,6 +295,8 @@ void _bmo_slot_copy(BMOpSlot slot_args_src[BMO_OP_MAX_SLOTS],
   else {
     slot_dst->data = slot_src->data;
   }
+
+  slot_dst->flag = slot_src->flag;
 }
 
 /*
@@ -312,6 +314,7 @@ void BMO_slot_float_set(BMOpSlot slot_args[BMO_OP_MAX_SLOTS], const char *slot_n
   }
 
   slot->data.f = f;
+  slot->flag |= BMO_OP_SLOT_FLAG_IS_SET;
 }
 
 void BMO_slot_int_set(BMOpSlot slot_args[BMO_OP_MAX_SLOTS], const char *slot_name, const int i)
@@ -323,6 +326,7 @@ void BMO_slot_int_set(BMOpSlot slot_args[BMO_OP_MAX_SLOTS], const char *slot_nam
   }
 
   slot->data.i = i;
+  slot->flag |= BMO_OP_SLOT_FLAG_IS_SET;
 }
 
 void BMO_slot_bool_set(BMOpSlot slot_args[BMO_OP_MAX_SLOTS], const char *slot_name, const bool i)
@@ -334,6 +338,7 @@ void BMO_slot_bool_set(BMOpSlot slot_args[BMO_OP_MAX_SLOTS], const char *slot_na
   }
 
   slot->data.i = i;
+  slot->flag |= BMO_OP_SLOT_FLAG_IS_SET;
 }
 
 void BMO_slot_mat_set(BMOperator *op,
@@ -362,6 +367,7 @@ void BMO_slot_mat_set(BMOperator *op,
 
     zero_m4(static_cast<float(*)[4]>(slot->data.p));
   }
+  slot->flag |= BMO_OP_SLOT_FLAG_IS_SET;
 }
 
 void BMO_slot_mat4_get(BMOpSlot slot_args[BMO_OP_MAX_SLOTS],
@@ -409,6 +415,7 @@ void BMO_slot_ptr_set(BMOpSlot slot_args[BMO_OP_MAX_SLOTS], const char *slot_nam
   }
 
   slot->data.p = p;
+  slot->flag |= BMO_OP_SLOT_FLAG_IS_SET;
 }
 
 void BMO_slot_vec_set(BMOpSlot slot_args[BMO_OP_MAX_SLOTS],
@@ -422,31 +429,51 @@ void BMO_slot_vec_set(BMOpSlot slot_args[BMO_OP_MAX_SLOTS],
   }
 
   copy_v3_v3(slot->data.vec, vec);
+  slot->flag |= BMO_OP_SLOT_FLAG_IS_SET;
 }
 
 float BMO_slot_float_get(BMOpSlot slot_args[BMO_OP_MAX_SLOTS], const char *slot_name)
+{
+  return BMO_slot_float_get_optional(slot_args, slot_name).value_or(0.0f);
+}
+
+std::optional<float> BMO_slot_float_get_optional(BMOpSlot slot_args[BMO_OP_MAX_SLOTS],
+                                                 const char *slot_name)
 {
   BMOpSlot *slot = BMO_slot_get(slot_args, slot_name);
   BLI_assert(slot->slot_type == BMO_OP_SLOT_FLT);
   if (!(slot->slot_type == BMO_OP_SLOT_FLT)) {
-    return 0.0f;
+    return std::nullopt;
+  }
+  if ((slot->flag & BMO_OP_SLOT_FLAG_IS_SET) == 0) {
+    return std::nullopt;
   }
 
   return slot->data.f;
 }
 
-int BMO_slot_int_get(BMOpSlot slot_args[BMO_OP_MAX_SLOTS], const char *slot_name)
+std::optional<int> BMO_slot_int_get_optional(BMOpSlot slot_args[BMO_OP_MAX_SLOTS],
+                                             const char *slot_name)
 {
   BMOpSlot *slot = BMO_slot_get(slot_args, slot_name);
   BLI_assert(slot->slot_type == BMO_OP_SLOT_INT);
   if (!(slot->slot_type == BMO_OP_SLOT_INT)) {
-    return 0;
+    return std::nullopt;
+  }
+  if ((slot->flag & BMO_OP_SLOT_FLAG_IS_SET) == 0) {
+    return std::nullopt;
   }
 
   return slot->data.i;
 }
 
-bool BMO_slot_bool_get(BMOpSlot slot_args[BMO_OP_MAX_SLOTS], const char *slot_name)
+int BMO_slot_int_get(BMOpSlot slot_args[BMO_OP_MAX_SLOTS], const char *slot_name)
+{
+  return BMO_slot_int_get_optional(slot_args, slot_name).value_or(0);
+}
+
+std::optional<bool> BMO_slot_bool_get_optional(BMOpSlot slot_args[BMO_OP_MAX_SLOTS],
+                                               const char *slot_name)
 {
   BMOpSlot *slot = BMO_slot_get(slot_args, slot_name);
   BLI_assert(slot->slot_type == BMO_OP_SLOT_BOOL);
@@ -456,6 +483,10 @@ bool BMO_slot_bool_get(BMOpSlot slot_args[BMO_OP_MAX_SLOTS], const char *slot_na
 
   return slot->data.i;
 }
+bool BMO_slot_bool_get(BMOpSlot slot_args[BMO_OP_MAX_SLOTS], const char *slot_name)
+{
+  return BMO_slot_bool_get_optional(slot_args, slot_name).value_or(false);
+}
 
 void *BMO_slot_as_arrayN(BMOpSlot slot_args[BMO_OP_MAX_SLOTS], const char *slot_name, int *len)
 {
diff --git a/source/blender/bmesh/operators/bmo_dissolve.cc b/source/blender/bmesh/operators/bmo_dissolve.cc
index f377a0dd269..b0ea5877f5a 100644
--- a/source/blender/bmesh/operators/bmo_dissolve.cc
+++ b/source/blender/bmesh/operators/bmo_dissolve.cc
@@ -387,7 +387,9 @@ void bmo_dissolve_edges_exec(BMesh *bm, BMOperator *op)
    * This lets the test ignore that tiny bit of math error so users won't notice. */
   const float angle_epsilon = RAD2DEGF(0.0001f);
 
-  const float angle_threshold = BMO_slot_float_get(op->slots_in, "angle_threshold");
+  /* When unset, don't limit dissolving vertices at all. */
+  const float angle_threshold =
+      BMO_slot_float_get_optional(op->slots_in, "angle_threshold").value_or(M_PI);
 
   /* Use verts when told to... except, do *not* use verts when angle_threshold is 0.0. */
   const bool use_verts = BMO_slot_bool_get(op->slots_in, "use_verts") &&